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/OpenJDK 21

Class Thread

java.lang.Object
java.lang.Thread
All Implemented Interfaces:
Runnable
Direct Known Subclasses:
ForkJoinWorkerThread
public class Thread extends Object implements Runnable
A thread is a thread of execution in a program. The Java virtual machine allows an application to have multiple threads of execution running concurrently.

Thread defines constructors and a Thread.Builder to create threads. Starting a thread schedules it to execute its run method. The newly started thread executes concurrently with the thread that caused it to start.

A thread terminates if either its run method completes normally, or if its run method completes abruptly and the appropriate uncaught exception handler completes normally or abruptly. With no code left to run, the thread has completed execution. The join method can be used to wait for a thread to terminate.

Threads have a unique identifier and a name. The identifier is generated when a Thread is created and cannot be changed. The thread name can be specified when creating a thread or can be changed at a later time.

Threads support ThreadLocal variables. These are variables that are local to a thread, meaning a thread can have a copy of a variable that is set to a value that is independent of the value set by other threads. Thread also supports InheritableThreadLocal variables that are thread local variables that are inherited at thread creation time from the parent Thread. Thread supports a special inheritable thread local for the thread context-class-loader.

Platform threads

Thread supports the creation of platform threads that are typically mapped 1:1 to kernel threads scheduled by the operating system. Platform threads will usually have a large stack and other resources that are maintained by the operating system. Platforms threads are suitable for executing all types of tasks but may be a limited resource.

Platform threads get an automatically generated thread name by default.

Platform threads are designated daemon or non-daemon threads. When the Java virtual machine starts up, there is usually one non-daemon thread (the thread that typically calls the application's main method). The shutdown sequence begins when all started non-daemon threads have terminated. Unstarted non-daemon threads do not prevent the shutdown sequence from beginning.

In addition to the daemon status, platform threads have a thread priority and are members of a thread group.

Virtual threads

Thread also supports the creation of virtual threads. Virtual threads are typically user-mode threads scheduled by the Java runtime rather than the operating system. Virtual threads will typically require few resources and a single Java virtual machine may support millions of virtual threads. Virtual threads are suitable for executing tasks that spend most of the time blocked, often waiting for I/O operations to complete. Virtual threads are not intended for long running CPU intensive operations.

Virtual threads typically employ a small set of platform threads used as carrier threads. Locking and I/O operations are examples of operations where a carrier thread may be re-scheduled from one virtual thread to another. Code executing in a virtual thread is not aware of the underlying carrier thread. The currentThread() method, used to obtain a reference to the current thread, will always return the Thread object for the virtual thread.

Virtual threads do not have a thread name by default. The getName method returns the empty string if a thread name is not set.

Virtual threads are daemon threads and so do not prevent the shutdown sequence from beginning. Virtual threads have a fixed thread priority that cannot be changed.

Creating and starting threads

Thread defines public constructors for creating platform threads and the start method to schedule threads to execute. Thread may be extended for customization and other advanced reasons although most applications should have little need to do this.

Thread defines a Thread.Builder API for creating and starting both platform and virtual threads. The following are examples that use the builder: 

  Runnable runnable = ...

  // Start a daemon thread to run a task
  Thread thread = Thread.ofPlatform().daemon().start(runnable);

  // Create an unstarted thread with name "duke", its start() method
  // must be invoked to schedule it to execute.
  Thread thread = Thread.ofPlatform().name("duke").unstarted(runnable);

  // A ThreadFactory that creates daemon threads named "worker-0", "worker-1", ...
  ThreadFactory factory = Thread.ofPlatform().daemon().name("worker-", 0).factory();

  // Start a virtual thread to run a task
  Thread thread = Thread.ofVirtual().start(runnable);

  // A ThreadFactory that creates virtual threads
  ThreadFactory factory = Thread.ofVirtual().factory();

Inheritance when creating threads

A Thread inherits its initial values of inheritable-thread-local variables (including the context class loader) from the parent thread values at the time that the child Thread is created. The 5-param constructor can be used to create a thread that does not inherit its initial values from the constructing thread. When using a Thread.Builder, the inheritInheritableThreadLocals method can be used to select if the initial values are inherited.

Platform threads inherit the daemon status, thread priority, and when not provided (or not selected by a security manager), the thread group.

Creating a platform thread captures the caller context to limit the permissions of the new thread when it executes code that performs a privileged action. The captured caller context is the new thread's "Inherited AccessControlContext". Creating a virtual thread does not capture the caller context; virtual threads have no permissions when executing code that performs a privileged action.

Unless otherwise specified, passing a null argument to a constructor or method in this class will cause a NullPointerException to be thrown.

Implementation Note:
In the JDK Reference Implementation, the virtual thread scheduler may be configured with the following system properties:
System properties
System property Description
jdk.virtualThreadScheduler.parallelism The number of platform threads available for scheduling virtual threads. It defaults to the number of available processors.
jdk.virtualThreadScheduler.maxPoolSize The maximum number of platform threads available to the scheduler. It defaults to 256.
Since:
1.0

Nested Class Summary

Modifier and Type Class Description
static interface  Thread.Builder
A builder for Thread and ThreadFactory objects.
static enum  Thread.State
A thread state.
static interface  Thread.UncaughtExceptionHandler
Interface for handlers invoked when a Thread abruptly terminates due to an uncaught exception.

Field Summary

Modifier and Type Field Description
static final int MAX_PRIORITY
The maximum priority that a thread can have.
static final int MIN_PRIORITY
The minimum priority that a thread can have.
static final int NORM_PRIORITY
The default priority that is assigned to a thread.

Constructor Summary

Constructor Description
Thread()
Initializes a new platform Thread.
Thread(Runnable task)
Initializes a new platform Thread.
Thread(Runnable task, String name)
Initializes a new platform Thread.
Thread(String name)
Initializes a new platform Thread.
Thread(ThreadGroup group, Runnable task)
Initializes a new platform Thread.
Thread(ThreadGroup group, Runnable task, String name)
Initializes a new platform Thread so that it has task as its run object, has the specified name as its name, and belongs to the thread group referred to by group.
Thread(ThreadGroup group, Runnable task, String name, long stackSize)
Initializes a new platform Thread so that it has task as its run object, has the specified name as its name, and belongs to the thread group referred to by group, and has the specified stack size.
Thread(ThreadGroup group, Runnable task, String name, long stackSize, boolean inheritInheritableThreadLocals)
Initializes a new platform Thread so that it has task as its run object, has the specified name as its name, belongs to the thread group referred to by group, has the specified stackSize, and inherits initial values for inheritable thread-local variables if inheritThreadLocals is true.
Thread(ThreadGroup group, String name)
Initializes a new platform Thread.

Method Summary

Modifier and Type Method Description
static int activeCount()
Returns an estimate of the number of live platform threads in the current thread's thread group and its subgroups.
final void checkAccess()
Deprecated, for removal: This API element is subject to removal in a future version.
This method is only useful in conjunction with the Security Manager, which is deprecated and subject to removal in a future release.
protected Object clone()
Throws CloneNotSupportedException as a Thread can not be meaningfully cloned.
int countStackFrames()
Deprecated, for removal: This API element is subject to removal in a future version.
This method was originally designed to count the number of stack frames but the results were never well-defined and it depended on thread-suspension.
static Thread currentThread()
Returns the Thread object for the current thread.
static void dumpStack()
Prints a stack trace of the current thread to the standard error stream.
static int enumerate(Thread[] tarray)
Copies into the specified array every live platform thread in the current thread's thread group and its subgroups.
static Map<Thread,StackTraceElement[]> getAllStackTraces()
Returns a map of stack traces for all live platform threads.
ClassLoader getContextClassLoader()
Returns the context ClassLoader for this thread.
static Thread.UncaughtExceptionHandler getDefaultUncaughtExceptionHandler()
Returns the default handler invoked when a thread abruptly terminates due to an uncaught exception.
long getId()
Deprecated.
This method is not final and may be overridden to return a value that is not the thread ID.
final String getName()
Returns this thread's name.
final int getPriority()
Returns this thread's priority.
StackTraceElement[] getStackTrace()
Returns an array of stack trace elements representing the stack dump of this thread.
Thread.State getState()
Returns the state of this thread.
final ThreadGroup getThreadGroup()
Returns the thread's thread group or null if the thread has terminated.
Thread.UncaughtExceptionHandler getUncaughtExceptionHandler()
Returns the handler invoked when this thread abruptly terminates due to an uncaught exception.
static boolean holdsLock(Object obj)
Returns true if and only if the current thread holds the monitor lock on the specified object.
void interrupt()
Interrupts this thread.
static boolean interrupted()
Tests whether the current thread has been interrupted.
final boolean isAlive()
Tests if this thread is alive.
final boolean isDaemon()
Tests if this thread is a daemon thread.
boolean isInterrupted()
Tests whether this thread has been interrupted.
final boolean isVirtual()
Returns true if this thread is a virtual thread.
final void join()
Waits for this thread to terminate.
final void join(long millis)
Waits at most millis milliseconds for this thread to terminate.
final void join(long millis, int nanos)
Waits at most millis milliseconds plus nanos nanoseconds for this thread to terminate.
final boolean join(Duration duration)
Waits for this thread to terminate for up to the given waiting duration.
static Thread.Builder.OfPlatform ofPlatform()
Returns a builder for creating a platform Thread or ThreadFactory that creates platform threads.
static Thread.Builder.OfVirtual ofVirtual()
Returns a builder for creating a virtual Thread or ThreadFactory that creates virtual threads.
static void onSpinWait()
Indicates that the caller is momentarily unable to progress, until the occurrence of one or more actions on the part of other activities.
final void resume()
Deprecated, for removal: This API element is subject to removal in a future version.
This method was originally specified to resume a thread suspended with suspend().
void run()
This method is run by the thread when it executes.
void setContextClassLoader(ClassLoader cl)
Sets the context ClassLoader for this thread.
final void setDaemon(boolean on)
Marks this thread as either a daemon or non-daemon thread.
static void setDefaultUncaughtExceptionHandler(Thread.UncaughtExceptionHandler ueh)
Set the default handler invoked when a thread abruptly terminates due to an uncaught exception, and no other handler has been defined for that thread.
final void setName(String name)
Changes the name of this thread to be equal to the argument name.
final void setPriority(int newPriority)
Changes the priority of this thread.
void setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler ueh)
Set the handler invoked when this thread abruptly terminates due to an uncaught exception.
static void sleep(long millis)
Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds, subject to the precision and accuracy of system timers and schedulers.
static void sleep(long millis, int nanos)
Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds plus the specified number of nanoseconds, subject to the precision and accuracy of system timers and schedulers.
static void sleep(Duration duration)
Causes the currently executing thread to sleep (temporarily cease execution) for the specified duration, subject to the precision and accuracy of system timers and schedulers.
void start()
Schedules this thread to begin execution.
static Thread startVirtualThread(Runnable task)
Creates a virtual thread to execute a task and schedules it to execute.
final void stop()
Deprecated, for removal: This API element is subject to removal in a future version.
This method was originally specified to "stop" a victim thread by causing the victim thread to throw a ThreadDeath.
final void suspend()
Deprecated, for removal: This API element is subject to removal in a future version.
This method was originally specified to suspend a thread.
final long threadId()
Returns the identifier of this Thread.
String toString()
Returns a string representation of this thread.
static void yield()
A hint to the scheduler that the current thread is willing to yield its current use of a processor.

Methods declared in class java.lang.Object

equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Details

MIN_PRIORITY

public static final int MIN_PRIORITY
The minimum priority that a thread can have.
See Also:

NORM_PRIORITY

public static final int NORM_PRIORITY
The default priority that is assigned to a thread.
See Also:

MAX_PRIORITY

public static final int MAX_PRIORITY
The maximum priority that a thread can have.
See Also:

Constructor Details

Thread

public Thread()
Initializes a new platform Thread. This constructor has the same effect as Thread (null, null, gname), where gname is a newly generated name. Automatically generated names are of the form "Thread-"+n, where n is an integer.

This constructor is only useful when extending Thread to override the run() method.

See Also:

Thread

public Thread(Runnable task)
Initializes a new platform Thread. This constructor has the same effect as Thread (null, task, gname), where gname is a newly generated name. Automatically generated names are of the form "Thread-"+n, where n is an integer.

For a non-null task, invoking this constructor directly is equivalent to: 

Thread.ofPlatform().unstarted(task);
Parameters:
task - the object whose run method is invoked when this thread is started. If null, this classes run method does nothing.
See Also:

Thread

public Thread(ThreadGroup group, Runnable task)
Initializes a new platform Thread. This constructor has the same effect as Thread (group, task, gname), where gname is a newly generated name. Automatically generated names are of the form "Thread-"+n, where n is an integer.

For a non-null group and task, invoking this constructor directly is equivalent to: 

Thread.ofPlatform().group(group).unstarted(task);
Parameters:
group - the thread group. If null and there is a security manager, the group is determined by SecurityManager.getThreadGroup(). If there is not a security manager or SecurityManager.getThreadGroup() returns null, the group is set to the current thread's thread group.
task - the object whose run method is invoked when this thread is started. If null, this thread's run method is invoked.
Throws:
SecurityException - if the current thread cannot create a thread in the specified thread group
See Also:

Thread

public Thread(String name)
Initializes a new platform Thread. This constructor has the same effect as Thread (null, null, name).

This constructor is only useful when extending Thread to override the run() method.

Parameters:
name - the name of the new thread
See Also:

Thread

public Thread(ThreadGroup group, String name)
Initializes a new platform Thread. This constructor has the same effect as Thread (group, null, name).

This constructor is only useful when extending Thread to override the run() method.

Parameters:
group - the thread group. If null and there is a security manager, the group is determined by SecurityManager.getThreadGroup(). If there is not a security manager or SecurityManager.getThreadGroup() returns null, the group is set to the current thread's thread group.
name - the name of the new thread
Throws:
SecurityException - if the current thread cannot create a thread in the specified thread group
See Also:

Thread

public Thread(Runnable task, String name)
Initializes a new platform Thread. This constructor has the same effect as Thread (null, task, name).

For a non-null task and name, invoking this constructor directly is equivalent to: 

Thread.ofPlatform().name(name).unstarted(task);
Parameters:
task - the object whose run method is invoked when this thread is started. If null, this thread's run method is invoked.
name - the name of the new thread
See Also:

Thread

public Thread(ThreadGroup group, Runnable task, String name)
Initializes a new platform Thread so that it has task as its run object, has the specified name as its name, and belongs to the thread group referred to by group.

If there is a security manager, its checkAccess method is invoked with the ThreadGroup as its argument.

In addition, its checkPermission method is invoked with the RuntimePermission("enableContextClassLoaderOverride") permission when invoked directly or indirectly by the constructor of a subclass which overrides the getContextClassLoader or setContextClassLoader methods.

The priority of the newly created thread is the smaller of priority of the thread creating it and the maximum permitted priority of the thread group. The method setPriority may be used to change the priority to a new value.

The newly created thread is initially marked as being a daemon thread if and only if the thread creating it is currently marked as a daemon thread. The method setDaemon may be used to change whether or not a thread is a daemon.

For a non-null group, task, and name, invoking this constructor directly is equivalent to: 

Thread.ofPlatform().group(group).name(name).unstarted(task);
Parameters:
group - the thread group. If null and there is a security manager, the group is determined by SecurityManager.getThreadGroup(). If there is not a security manager or SecurityManager.getThreadGroup() returns null, the group is set to the current thread's thread group.
task - the object whose run method is invoked when this thread is started. If null, this thread's run method is invoked.
name - the name of the new thread
Throws:
SecurityException - if the current thread cannot create a thread in the specified thread group or cannot override the context class loader methods.
See Also:

Thread

public Thread(ThreadGroup group, Runnable task, String name, long stackSize)
Initializes a new platform Thread so that it has task as its run object, has the specified name as its name, and belongs to the thread group referred to by group, and has the specified stack size.

This constructor is identical to Thread(ThreadGroup,Runnable,String) with the exception of the fact that it allows the thread stack size to be specified. The stack size is the approximate number of bytes of address space that the virtual machine is to allocate for this thread's stack. The effect of the stackSize parameter, if any, is highly platform dependent.

On some platforms, specifying a higher value for the stackSize parameter may allow a thread to achieve greater recursion depth before throwing a StackOverflowError. Similarly, specifying a lower value may allow a greater number of threads to exist concurrently without throwing an OutOfMemoryError (or other internal error). The details of the relationship between the value of the stackSize parameter and the maximum recursion depth and concurrency level are platform-dependent. On some platforms, the value of the stackSize parameter may have no effect whatsoever.

The virtual machine is free to treat the stackSize parameter as a suggestion. If the specified value is unreasonably low for the platform, the virtual machine may instead use some platform-specific minimum value; if the specified value is unreasonably high, the virtual machine may instead use some platform-specific maximum. Likewise, the virtual machine is free to round the specified value up or down as it sees fit (or to ignore it completely).

Specifying a value of zero for the stackSize parameter will cause this constructor to behave exactly like the Thread(ThreadGroup, Runnable, String) constructor.

Due to the platform-dependent nature of the behavior of this constructor, extreme care should be exercised in its use. The thread stack size necessary to perform a given computation will likely vary from one JRE implementation to another. In light of this variation, careful tuning of the stack size parameter may be required, and the tuning may need to be repeated for each JRE implementation on which an application is to run.

Implementation note: Java platform implementers are encouraged to document their implementation's behavior with respect to the stackSize parameter.

For a non-null group, task, and name, invoking this constructor directly is equivalent to: 

Thread.ofPlatform().group(group).name(name).stackSize(stackSize).unstarted(task);
Parameters:
group - the thread group. If null and there is a security manager, the group is determined by SecurityManager.getThreadGroup(). If there is not a security manager or SecurityManager.getThreadGroup() returns null, the group is set to the current thread's thread group.
task - the object whose run method is invoked when this thread is started. If null, this thread's run method is invoked.
name - the name of the new thread
stackSize - the desired stack size for the new thread, or zero to indicate that this parameter is to be ignored.
Throws:
SecurityException - if the current thread cannot create a thread in the specified thread group
Since:
1.4
See Also:

Thread

public Thread(ThreadGroup group, Runnable task, String name, long stackSize, boolean inheritInheritableThreadLocals)
Initializes a new platform Thread so that it has task as its run object, has the specified name as its name, belongs to the thread group referred to by group, has the specified stackSize, and inherits initial values for inheritable thread-local variables if inheritThreadLocals is true.

This constructor is identical to Thread(ThreadGroup,Runnable,String,long) with the added ability to suppress, or not, the inheriting of initial values for inheritable thread-local variables from the constructing thread. This allows for finer grain control over inheritable thread-locals. Care must be taken when passing a value of false for inheritThreadLocals, as it may lead to unexpected behavior if the new thread executes code that expects a specific thread-local value to be inherited.

Specifying a value of true for the inheritThreadLocals parameter will cause this constructor to behave exactly like the Thread(ThreadGroup, Runnable, String, long) constructor.

For a non-null group, task, and name, invoking this constructor directly is equivalent to: 

Thread.ofPlatform()
      .group(group)
      .name(name)
      .stackSize(stackSize)
      .inheritInheritableThreadLocals(inheritInheritableThreadLocals)
      .unstarted(task);
Parameters:
group - the thread group. If null and there is a security manager, the group is determined by SecurityManager.getThreadGroup(). If there is not a security manager or SecurityManager.getThreadGroup() returns null, the group is set to the current thread's thread group.
task - the object whose run method is invoked when this thread is started. If null, this thread's run method is invoked.
name - the name of the new thread
stackSize - the desired stack size for the new thread, or zero to indicate that this parameter is to be ignored
inheritInheritableThreadLocals - if true, inherit initial values for inheritable thread-locals from the constructing thread, otherwise no initial values are inherited
Throws:
SecurityException - if the current thread cannot create a thread in the specified thread group
Since:
9
See Also:

Method Details

currentThread

public static Thread currentThread()
Returns the Thread object for the current thread.
Returns:
the current thread

yield

public static void yield()
A hint to the scheduler that the current thread is willing to yield its current use of a processor. The scheduler is free to ignore this hint.

Yield is a heuristic attempt to improve relative progression between threads that would otherwise over-utilise a CPU. Its use should be combined with detailed profiling and benchmarking to ensure that it actually has the desired effect.

It is rarely appropriate to use this method. It may be useful for debugging or testing purposes, where it may help to reproduce bugs due to race conditions. It may also be useful when designing concurrency control constructs such as the ones in the java.util.concurrent.locks package.

sleep

public static void sleep(long millis) throws InterruptedException
Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds, subject to the precision and accuracy of system timers and schedulers. The thread does not lose ownership of any monitors.
Parameters:
millis - the length of time to sleep in milliseconds
Throws:
IllegalArgumentException - if the value of millis is negative
InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

sleep

public static void sleep(long millis, int nanos) throws InterruptedException
Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds plus the specified number of nanoseconds, subject to the precision and accuracy of system timers and schedulers. The thread does not lose ownership of any monitors.
Parameters:
millis - the length of time to sleep in milliseconds
nanos - 0-999999 additional nanoseconds to sleep
Throws:
IllegalArgumentException - if the value of millis is negative, or the value of nanos is not in the range 0-999999
InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

sleep

public static void sleep(Duration duration) throws InterruptedException
Causes the currently executing thread to sleep (temporarily cease execution) for the specified duration, subject to the precision and accuracy of system timers and schedulers. This method is a no-op if the duration is negative.
Parameters:
duration - the duration to sleep
Throws:
InterruptedException - if the current thread is interrupted while sleeping. The interrupted status of the current thread is cleared when this exception is thrown.
Since:
19

onSpinWait

public static void onSpinWait()
Indicates that the caller is momentarily unable to progress, until the occurrence of one or more actions on the part of other activities. By invoking this method within each iteration of a spin-wait loop construct, the calling thread indicates to the runtime that it is busy-waiting. The runtime may take action to improve the performance of invoking spin-wait loop constructions.
API Note:
As an example consider a method in a class that spins in a loop until some flag is set outside of that method. A call to the onSpinWait method should be placed inside the spin loop. 
    class EventHandler {
        volatile boolean eventNotificationNotReceived;
        void waitForEventAndHandleIt() {
            while ( eventNotificationNotReceived ) {
                Thread.onSpinWait();
            }
            readAndProcessEvent();
        }

        void readAndProcessEvent() {
            // Read event from some source and process it
             . . .
        }
    }

The code above would remain correct even if the onSpinWait method was not called at all. However on some architectures the Java Virtual Machine may issue the processor instructions to address such code patterns in a more beneficial way.

Since:
9

ofPlatform

public static Thread.Builder.OfPlatform ofPlatform()
Returns a builder for creating a platform Thread or ThreadFactory that creates platform threads.

Interaction with security manager when creating platform threads

Creating a platform thread when there is a security manager set will invoke the security manager's checkAccess(ThreadGroup) method with the thread's thread group. If the thread group has not been set with the OfPlatform.group method then the security manager's getThreadGroup method will be invoked first to select the thread group. If the security manager getThreadGroup method returns null then the thread group of the constructing thread is used.

API Note:
The following are examples using the builder: 
  // Start a daemon thread to run a task
  Thread thread = Thread.ofPlatform().daemon().start(runnable);

  // Create an unstarted thread with name "duke", its start() method
  // must be invoked to schedule it to execute.
  Thread thread = Thread.ofPlatform().name("duke").unstarted(runnable);

  // A ThreadFactory that creates daemon threads named "worker-0", "worker-1", ...
  ThreadFactory factory = Thread.ofPlatform().daemon().name("worker-", 0).factory();
Returns:
A builder for creating Thread or ThreadFactory objects.
Since:
21

ofVirtual

public static Thread.Builder.OfVirtual ofVirtual()
Returns a builder for creating a virtual Thread or ThreadFactory that creates virtual threads.
API Note:
The following are examples using the builder: 
  // Start a virtual thread to run a task.
  Thread thread = Thread.ofVirtual().start(runnable);

  // A ThreadFactory that creates virtual threads
  ThreadFactory factory = Thread.ofVirtual().factory();
Returns:
A builder for creating Thread or ThreadFactory objects.
Since:
21

clone

protected Object clone() throws CloneNotSupportedException
Throws CloneNotSupportedException as a Thread can not be meaningfully cloned. Construct a new Thread instead.
Overrides:
clone in class Object
Returns:
a clone of this instance.
Throws:
CloneNotSupportedException - always
See Also:

startVirtualThread

public static Thread startVirtualThread(Runnable task)
Creates a virtual thread to execute a task and schedules it to execute.

This method is equivalent to: 

Thread.ofVirtual().start(task);
Parameters:
task - the object to run when the thread executes
Returns:
a new, and started, virtual thread
Since:
21
See Also:

isVirtual

public final boolean isVirtual()
Returns true if this thread is a virtual thread. A virtual thread is scheduled by the Java virtual machine rather than the operating system.
Returns:
true if this thread is a virtual thread
Since:
21

start

public void start()
Schedules this thread to begin execution. The thread will execute independently of the current thread.

A thread can be started at most once. In particular, a thread can not be restarted after it has terminated.

Throws:
IllegalThreadStateException - if the thread was already started

run

public void run()
This method is run by the thread when it executes. Subclasses of Thread may override this method.

This method is not intended to be invoked directly. If this thread is a platform thread created with a Runnable task then invoking this method will invoke the task's run method. If this thread is a virtual thread then invoking this method directly does nothing.

Specified by:
run in interface Runnable
Implementation Requirements:
The default implementation executes the Runnable task that the Thread was created with. If the thread was created without a task then this method does nothing.

stop

@Deprecated(since="1.2", forRemoval=true) public final void stop()
Deprecated, for removal: This API element is subject to removal in a future version.
This method was originally specified to "stop" a victim thread by causing the victim thread to throw a ThreadDeath. It was inherently unsafe. Stopping a thread caused it to unlock all of the monitors that it had locked (as a natural consequence of the ThreadDeath exception propagating up the stack). If any of the objects previously protected by these monitors were in an inconsistent state, the damaged objects became visible to other threads, potentially resulting in arbitrary behavior. Usages of stop should be replaced by code that simply modifies some variable to indicate that the target thread should stop running. The target thread should check this variable regularly, and return from its run method in an orderly fashion if the variable indicates that it is to stop running. If the target thread waits for long periods (on a condition variable, for example), the interrupt method should be used to interrupt the wait. For more information, see Why are Thread.stop, Thread.suspend and Thread.resume Deprecated?.
Throws UnsupportedOperationException.
Throws:
UnsupportedOperationException - always

interrupt

public void interrupt()
Interrupts this thread.

Unless the current thread is interrupting itself, which is always permitted, the checkAccess method of this thread is invoked, which may cause a SecurityException to be thrown.

If this thread is blocked in an invocation of the wait(), wait(long), or wait(long, int) methods of the Object class, or of the join(), join(long), join(long, int), sleep(long), or sleep(long, int) methods of this class, then its interrupt status will be cleared and it will receive an InterruptedException.

If this thread is blocked in an I/O operation upon an InterruptibleChannel then the channel will be closed, the thread's interrupt status will be set, and the thread will receive a ClosedByInterruptException.

If this thread is blocked in a Selector then the thread's interrupt status will be set and it will return immediately from the selection operation, possibly with a non-zero value, just as if the selector's wakeup method were invoked.

If none of the previous conditions hold then this thread's interrupt status will be set.

Interrupting a thread that is not alive need not have any effect.

Implementation Note:
In the JDK Reference Implementation, interruption of a thread that is not alive still records that the interrupt request was made and will report it via interrupted() and isInterrupted().
Throws:
SecurityException - if the current thread cannot modify this thread

interrupted

public static boolean interrupted()
Tests whether the current thread has been interrupted. The interrupted status of the thread is cleared by this method. In other words, if this method were to be called twice in succession, the second call would return false (unless the current thread were interrupted again, after the first call had cleared its interrupted status and before the second call had examined it).
Returns:
true if the current thread has been interrupted; false otherwise.
See Also:

isInterrupted

public boolean isInterrupted()
Tests whether this thread has been interrupted. The interrupted status of the thread is unaffected by this method.
Returns:
true if this thread has been interrupted; false otherwise.
See Also:

isAlive

public final boolean isAlive()
Tests if this thread is alive. A thread is alive if it has been started and has not yet terminated.
Returns:
true if this thread is alive; false otherwise.

suspend

@Deprecated(since="1.2", forRemoval=true) public final void suspend()
Deprecated, for removal: This API element is subject to removal in a future version.
This method was originally specified to suspend a thread. It was inherently deadlock-prone. If the target thread held a lock on a monitor protecting a critical system resource when it was suspended, no thread could access the resource until the target thread was resumed. If the thread intending to resume the target thread attempted to lock the monitor prior to calling resume, deadlock would result. Such deadlocks typically manifested themselves as "frozen" processes. For more information, see Why are Thread.stop, Thread.suspend and Thread.resume Deprecated?.
Throws UnsupportedOperationException.
Throws:
UnsupportedOperationException - always

resume

@Deprecated(since="1.2", forRemoval=true) public final void resume()
Deprecated, for removal: This API element is subject to removal in a future version.
This method was originally specified to resume a thread suspended with suspend(). Suspending a thread was inherently deadlock-prone. For more information, see Why are Thread.stop, Thread.suspend and Thread.resume Deprecated?.
Throws UnsupportedOperationException.
Throws:
UnsupportedOperationException - always

setPriority

public final void setPriority(int newPriority)
Changes the priority of this thread. For platform threads, the priority is set to the smaller of the specified newPriority and the maximum permitted priority of the thread's thread group. The priority of a virtual thread is always NORM_PRIORITY and newPriority is ignored.
Parameters:
newPriority - the new thread priority
Throws:
IllegalArgumentException - if the priority is not in the range MIN_PRIORITY to MAX_PRIORITY.
SecurityException - if checkAccess() determines that the current thread cannot modify this thread
See Also:

getPriority

public final int getPriority()
Returns this thread's priority.

The priority of a virtual thread is always NORM_PRIORITY.

Returns:
this thread's priority.
See Also:

setName

public final void setName(String name)
Changes the name of this thread to be equal to the argument name.

First the checkAccess method of this thread is called with no arguments. This may result in throwing a SecurityException.

Implementation Note:
In the JDK Reference Implementation, if this thread is the current thread, and it's a platform thread that was not attached to the VM with the Java Native Interface AttachCurrentThread function, then this method will set the operating system thread name. This may be useful for debugging and troubleshooting purposes.
Parameters:
name - the new name for this thread.
Throws:
SecurityException - if the current thread cannot modify this thread.
External Specifications
See Also:

getName

public final String getName()
Returns this thread's name.
Returns:
this thread's name.
See Also:

getThreadGroup

public final ThreadGroup getThreadGroup()
Returns the thread's thread group or null if the thread has terminated.

The thread group returned for a virtual thread is the special ThreadGroup for virtual threads.

Returns:
this thread's thread group or null

activeCount

public static int activeCount()
Returns an estimate of the number of live platform threads in the current thread's thread group and its subgroups. Virtual threads are not included in the estimate.

The value returned is only an estimate because the number of threads may change dynamically while this method traverses internal data structures, and might be affected by the presence of certain system threads. This method is intended primarily for debugging and monitoring purposes.

Returns:
an estimate of the number of live platform threads in the current thread's thread group and in any other thread group that has the current thread's thread group as an ancestor

enumerate

public static int enumerate(Thread[] tarray)
Copies into the specified array every live platform thread in the current thread's thread group and its subgroups. This method simply invokes the ThreadGroup.enumerate(Thread[]) method of the current thread's thread group. Virtual threads are not enumerated by this method.

An application might use the activeCount method to get an estimate of how big the array should be, however if the array is too short to hold all the threads, the extra threads are silently ignored. If it is critical to obtain every live thread in the current thread's thread group and its subgroups, the invoker should verify that the returned int value is strictly less than the length of tarray.

Due to the inherent race condition in this method, it is recommended that the method only be used for debugging and monitoring purposes.

Parameters:
tarray - an array into which to put the list of threads
Returns:
the number of threads put into the array
Throws:
SecurityException - if ThreadGroup.checkAccess() determines that the current thread cannot access its thread group

countStackFrames

@Deprecated(since="1.2", forRemoval=true) public int countStackFrames()
Deprecated, for removal: This API element is subject to removal in a future version.
This method was originally designed to count the number of stack frames but the results were never well-defined and it depended on thread-suspension. This method is subject to removal in a future version of Java SE.
Throws UnsupportedOperationException.
Returns:
nothing
See Also:

join

public final void join(long millis) throws InterruptedException
Waits at most millis milliseconds for this thread to terminate. A timeout of 0 means to wait forever. This method returns immediately, without waiting, if the thread has not been started.
Implementation Note:
For platform threads, the implementation uses a loop of this.wait calls conditioned on this.isAlive. As a thread terminates the this.notifyAll method is invoked. It is recommended that applications not use wait, notify, or notifyAll on Thread instances.
Parameters:
millis - the time to wait in milliseconds
Throws:
IllegalArgumentException - if the value of millis is negative
InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

join

public final void join(long millis, int nanos) throws InterruptedException
Waits at most millis milliseconds plus nanos nanoseconds for this thread to terminate. If both arguments are 0, it means to wait forever. This method returns immediately, without waiting, if the thread has not been started.
Implementation Note:
For platform threads, the implementation uses a loop of this.wait calls conditioned on this.isAlive. As a thread terminates the this.notifyAll method is invoked. It is recommended that applications not use wait, notify, or notifyAll on Thread instances.
Parameters:
millis - the time to wait in milliseconds
nanos - 0-999999 additional nanoseconds to wait
Throws:
IllegalArgumentException - if the value of millis is negative, or the value of nanos is not in the range 0-999999
InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

join

public final void join() throws InterruptedException
Waits for this thread to terminate.

An invocation of this method behaves in exactly the same way as the invocation

join(0)
Throws:
InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

join

public final boolean join(Duration duration) throws InterruptedException
Waits for this thread to terminate for up to the given waiting duration.

This method does not wait if the duration to wait is less than or equal to zero. In this case, the method just tests if the thread has terminated.

Parameters:
duration - the maximum duration to wait
Returns:
true if the thread has terminated, false if the thread has not terminated
Throws:
InterruptedException - if the current thread is interrupted while waiting. The interrupted status of the current thread is cleared when this exception is thrown.
IllegalThreadStateException - if this thread has not been started.
Since:
19

dumpStack

public static void dumpStack()
Prints a stack trace of the current thread to the standard error stream. This method is useful for debugging.

setDaemon

public final void setDaemon(boolean on)
Marks this thread as either a daemon or non-daemon thread. The shutdown sequence begins when all started non-daemon threads have terminated.

The daemon status of a virtual thread is always true and cannot be changed by this method to false.

This method must be invoked before the thread is started. The behavior of this method when the thread has terminated is not specified.

Parameters:
on - if true, marks this thread as a daemon thread
Throws:
IllegalArgumentException - if this is a virtual thread and on is false
IllegalThreadStateException - if this thread is alive
SecurityException - if checkAccess() determines that the current thread cannot modify this thread

isDaemon

public final boolean isDaemon()
Tests if this thread is a daemon thread. The daemon status of a virtual thread is always true.
Returns:
true if this thread is a daemon thread; false otherwise.
See Also:

checkAccess

@Deprecated(since="17", forRemoval=true) public final void checkAccess()
Deprecated, for removal: This API element is subject to removal in a future version.
This method is only useful in conjunction with the Security Manager, which is deprecated and subject to removal in a future release. Consequently, this method is also deprecated and subject to removal. There is no replacement for the Security Manager or this method.
Determines if the currently running thread has permission to modify this thread.

If there is a security manager, its checkAccess method is called with this thread as its argument. This may result in throwing a SecurityException.

Throws:
SecurityException - if the current thread is not allowed to access this thread.
See Also:

toString

public String toString()
Returns a string representation of this thread. The string representation will usually include the thread's identifier and name. The default implementation for platform threads includes the thread's identifier, name, priority, and the name of the thread group.
Overrides:
toString in class Object
Returns:
a string representation of this thread.

getContextClassLoader

public ClassLoader getContextClassLoader()
Returns the context ClassLoader for this thread. The context ClassLoader may be set by the creator of the thread for use by code running in this thread when loading classes and resources. If not set, the default is to inherit the context class loader from the parent thread.

The context ClassLoader of the primordial thread is typically set to the class loader used to load the application.

Returns:
the context ClassLoader for this thread, or null indicating the system class loader (or, failing that, the bootstrap class loader)
Throws:
SecurityException - if a security manager is present, and the caller's class loader is not null and is not the same as or an ancestor of the context class loader, and the caller does not have the RuntimePermission("getClassLoader")
Since:
1.2

setContextClassLoader

public void setContextClassLoader(ClassLoader cl)
Sets the context ClassLoader for this thread.

The context ClassLoader may be set by the creator of the thread for use by code running in this thread when loading classes and resources.

If a security manager is present, its checkPermission method is invoked with a RuntimePermission ("setContextClassLoader") permission to see if setting the context ClassLoader is permitted.

Parameters:
cl - the context ClassLoader for this Thread, or null indicating the system class loader (or, failing that, the bootstrap class loader)
Throws:
SecurityException - if the current thread cannot set the context ClassLoader
Since:
1.2

holdsLock

public static boolean holdsLock(Object obj)
Returns true if and only if the current thread holds the monitor lock on the specified object.

This method is designed to allow a program to assert that the current thread already holds a specified lock: 

     assert Thread.holdsLock(obj);
Parameters:
obj - the object on which to test lock ownership
Returns:
true if the current thread holds the monitor lock on the specified object.
Since:
1.4

getStackTrace

public StackTraceElement[] getStackTrace()
Returns an array of stack trace elements representing the stack dump of this thread. This method will return a zero-length array if this thread has not started, has started but has not yet been scheduled to run by the system, or has terminated. If the returned array is of non-zero length then the first element of the array represents the top of the stack, which is the most recent method invocation in the sequence. The last element of the array represents the bottom of the stack, which is the least recent method invocation in the sequence.

If there is a security manager, and this thread is not the current thread, then the security manager's checkPermission method is called with a RuntimePermission("getStackTrace") permission to see if it's ok to get the stack trace.

Some virtual machines may, under some circumstances, omit one or more stack frames from the stack trace. In the extreme case, a virtual machine that has no stack trace information concerning this thread is permitted to return a zero-length array from this method.

Returns:
an array of StackTraceElement, each represents one stack frame.
Throws:
SecurityException - if a security manager exists and its checkPermission method doesn't allow getting the stack trace of thread.
Since:
1.5
See Also:

getAllStackTraces

public static Map<Thread,StackTraceElement[]> getAllStackTraces()
Returns a map of stack traces for all live platform threads. The map does not include virtual threads. The map keys are threads and each map value is an array of StackTraceElement that represents the stack dump of the corresponding Thread. The returned stack traces are in the format specified for the getStackTrace method.

The threads may be executing while this method is called. The stack trace of each thread only represents a snapshot and each stack trace may be obtained at different time. A zero-length array will be returned in the map value if the virtual machine has no stack trace information about a thread.

If there is a security manager, then the security manager's checkPermission method is called with a RuntimePermission("getStackTrace") permission as well as RuntimePermission("modifyThreadGroup") permission to see if it is ok to get the stack trace of all threads.

Returns:
a Map from Thread to an array of StackTraceElement that represents the stack trace of the corresponding thread.
Throws:
SecurityException - if a security manager exists and its checkPermission method doesn't allow getting the stack trace of thread.
Since:
1.5
See Also:

getId

@Deprecated(since="19") public long getId()
Deprecated.
This method is not final and may be overridden to return a value that is not the thread ID. Use threadId() instead.
Returns the identifier of this Thread. The thread ID is a positive long number generated when this thread was created. The thread ID is unique and remains unchanged during its lifetime.
Returns:
this thread's ID
Since:
1.5

threadId

public final long threadId()
Returns the identifier of this Thread. The thread ID is a positive long number generated when this thread was created. The thread ID is unique and remains unchanged during its lifetime.
Returns:
this thread's ID
Since:
19

getState

public Thread.State getState()
Returns the state of this thread. This method is designed for use in monitoring of the system state, not for synchronization control.
Returns:
this thread's state.
Since:
1.5

setDefaultUncaughtExceptionHandler

public static void setDefaultUncaughtExceptionHandler(Thread.UncaughtExceptionHandler ueh)
Set the default handler invoked when a thread abruptly terminates due to an uncaught exception, and no other handler has been defined for that thread.

Uncaught exception handling is controlled first by the thread, then by the thread's ThreadGroup object and finally by the default uncaught exception handler. If the thread does not have an explicit uncaught exception handler set, and the thread's thread group (including parent thread groups) does not specialize its uncaughtException method, then the default handler's uncaughtException method will be invoked.

By setting the default uncaught exception handler, an application can change the way in which uncaught exceptions are handled (such as logging to a specific device, or file) for those threads that would already accept whatever "default" behavior the system provided.

Note that the default uncaught exception handler should not usually defer to the thread's ThreadGroup object, as that could cause infinite recursion.

Parameters:
ueh - the object to use as the default uncaught exception handler. If null then there is no default handler.
Throws:
SecurityException - if a security manager is present and it denies RuntimePermission("setDefaultUncaughtExceptionHandler")
Since:
1.5
See Also:

getDefaultUncaughtExceptionHandler

public static Thread.UncaughtExceptionHandler getDefaultUncaughtExceptionHandler()
Returns the default handler invoked when a thread abruptly terminates due to an uncaught exception. If the returned value is null, there is no default.
Returns:
the default uncaught exception handler for all threads
Since:
1.5
See Also:

getUncaughtExceptionHandler

public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler()
Returns the handler invoked when this thread abruptly terminates due to an uncaught exception. If this thread has not had an uncaught exception handler explicitly set then this thread's ThreadGroup object is returned, unless this thread has terminated, in which case null is returned.
Returns:
the uncaught exception handler for this thread
Since:
1.5

setUncaughtExceptionHandler

public void setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler ueh)
Set the handler invoked when this thread abruptly terminates due to an uncaught exception.

A thread can take full control of how it responds to uncaught exceptions by having its uncaught exception handler explicitly set. If no such handler is set then the thread's ThreadGroup object acts as its handler.

Parameters:
ueh - the object to use as this thread's uncaught exception handler. If null then this thread has no explicit handler.
Throws:
SecurityException - if the current thread is not allowed to modify this thread.
Since:
1.5
See Also:

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Thread.html