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Class ProcessBuilder

java.lang.Object
java.lang.ProcessBuilder
public final class ProcessBuilder extends Object
This class is used to create operating system processes.

Each ProcessBuilder instance manages a collection of process attributes. The start() method creates a new Process instance with those attributes. The start() method can be invoked repeatedly from the same instance to create new subprocesses with identical or related attributes.

The startPipeline method can be invoked to create a pipeline of new processes that send the output of each process directly to the next process. Each process has the attributes of its respective ProcessBuilder.

Each process builder manages these process attributes:

  • a command, a list of strings which signifies the external program file to be invoked and its arguments, if any. Which string lists represent a valid operating system command is system-dependent. For example, it is common for each conceptual argument to be an element in this list, but there are operating systems where programs are expected to tokenize command line strings themselves - on such a system a Java implementation might require commands to contain exactly two elements.
  • an environment, which is a system-dependent mapping from variables to values. The initial value is a copy of the environment of the current process (see System.getenv()).
  • a working directory. The default value is the current working directory of the current process, usually the directory named by the system property user.dir.
  • a source of standard input. By default, the subprocess reads input from a pipe. Java code can access this pipe via the output stream returned by Process.getOutputStream(). However, standard input may be redirected to another source using redirectInput. In this case, Process.getOutputStream() will return a null output stream, for which:
    • the write methods always throw IOException
    • the close method does nothing
  • a destination for standard output and standard error. By default, the subprocess writes standard output and standard error to pipes. Java code can access these pipes via the input streams returned by Process.getInputStream() and Process.getErrorStream(). However, standard output and standard error may be redirected to other destinations using redirectOutput and redirectError. In this case, Process.getInputStream() and/or Process.getErrorStream() will return a null input stream, for which:
    • the read methods always return -1
    • the available method always returns 0
    • the close method does nothing
  • a redirectErrorStream property. Initially, this property is false, meaning that the standard output and error output of a subprocess are sent to two separate streams, which can be accessed using the Process.getInputStream() and Process.getErrorStream() methods.

    If the value is set to true, then:

    • standard error is merged with the standard output and always sent to the same destination (this makes it easier to correlate error messages with the corresponding output)
    • the common destination of standard error and standard output can be redirected using redirectOutput
    • any redirection set by the redirectError method is ignored when creating a subprocess
    • the stream returned from Process.getErrorStream() will always be a null input stream

Modifying a process builder's attributes will affect processes subsequently started by that object's start() method, but will never affect previously started processes or the Java process itself.

Most error checking is performed by the start() method. It is possible to modify the state of an object so that start() will fail. For example, setting the command attribute to an empty list will not throw an exception unless start() is invoked.

Note that this class is not synchronized. If multiple threads access a ProcessBuilder instance concurrently, and at least one of the threads modifies one of the attributes structurally, it must be synchronized externally.

Starting a new process which uses the default working directory and environment is easy: 

 
 Process p = new ProcessBuilder("myCommand", "myArg").start();

Here is an example that starts a process with a modified working directory and environment, and redirects standard output and error to be appended to a log file: 

 
 ProcessBuilder pb =
   new ProcessBuilder("myCommand", "myArg1", "myArg2");
 Map<String, String> env = pb.environment();
 env.put("VAR1", "myValue");
 env.remove("OTHERVAR");
 env.put("VAR2", env.get("VAR1") + "suffix");
 pb.directory(new File("myDir"));
 File log = new File("log");
 pb.redirectErrorStream(true);
 pb.redirectOutput(Redirect.appendTo(log));
 Process p = pb.start();
 assert pb.redirectInput() == Redirect.PIPE;
 assert pb.redirectOutput().file() == log;
 assert p.getInputStream().read() == -1;

To start a process with an explicit set of environment variables, first call Map.clear() before adding environment variables.

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

Since:
1.5

Nested Class Summary

Modifier and Type Class Description
static class  ProcessBuilder.Redirect
Represents a source of subprocess input or a destination of subprocess output.

Constructor Summary

Constructor Description
ProcessBuilder(String... command)
Constructs a process builder with the specified operating system program and arguments.
ProcessBuilder(List<String> command)
Constructs a process builder with the specified operating system program and arguments.

Method Summary

Modifier and Type Method Description
List<String> command()
Returns this process builder's operating system program and arguments.
ProcessBuilder command(String... command)
Sets this process builder's operating system program and arguments.
ProcessBuilder command(List<String> command)
Sets this process builder's operating system program and arguments.
File directory()
Returns this process builder's working directory.
ProcessBuilder directory(File directory)
Sets this process builder's working directory.
Map<String,String> environment()
Returns a string map view of this process builder's environment.
ProcessBuilder inheritIO()
Sets the source and destination for subprocess standard I/O to be the same as those of the current Java process.
ProcessBuilder.Redirect redirectError()
Returns this process builder's standard error destination.
ProcessBuilder redirectError(File file)
Sets this process builder's standard error destination to a file.
ProcessBuilder redirectError(ProcessBuilder.Redirect destination)
Sets this process builder's standard error destination.
boolean redirectErrorStream()
Tells whether this process builder merges standard error and standard output.
ProcessBuilder redirectErrorStream(boolean redirectErrorStream)
Sets this process builder's redirectErrorStream property.
ProcessBuilder.Redirect redirectInput()
Returns this process builder's standard input source.
ProcessBuilder redirectInput(File file)
Sets this process builder's standard input source to a file.
ProcessBuilder redirectInput(ProcessBuilder.Redirect source)
Sets this process builder's standard input source.
ProcessBuilder.Redirect redirectOutput()
Returns this process builder's standard output destination.
ProcessBuilder redirectOutput(File file)
Sets this process builder's standard output destination to a file.
ProcessBuilder redirectOutput(ProcessBuilder.Redirect destination)
Sets this process builder's standard output destination.
Process start()
Starts a new process using the attributes of this process builder.
static List<Process> startPipeline(List<ProcessBuilder> builders)
Starts a Process for each ProcessBuilder, creating a pipeline of processes linked by their standard output and standard input streams.

Methods declared in class java.lang.Object

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

Constructor Details

ProcessBuilder

public ProcessBuilder(List<String> command)
Constructs a process builder with the specified operating system program and arguments. This constructor does not make a copy of the command list. Subsequent updates to the list will be reflected in the state of the process builder. It is not checked whether command corresponds to a valid operating system command.
Parameters:
command - the list containing the program and its arguments

ProcessBuilder

public ProcessBuilder(String... command)
Constructs a process builder with the specified operating system program and arguments. This is a convenience constructor that sets the process builder's command to a string list containing the same strings as the command array, in the same order. It is not checked whether command corresponds to a valid operating system command.
Parameters:
command - a string array containing the program and its arguments

Method Details

command

public ProcessBuilder command(List<String> command)
Sets this process builder's operating system program and arguments. This method does not make a copy of the command list. Subsequent updates to the list will be reflected in the state of the process builder. It is not checked whether command corresponds to a valid operating system command.
Parameters:
command - the list containing the program and its arguments
Returns:
this process builder

command

public ProcessBuilder command(String... command)
Sets this process builder's operating system program and arguments. This is a convenience method that sets the command to a string list containing the same strings as the command array, in the same order. It is not checked whether command corresponds to a valid operating system command.
Parameters:
command - a string array containing the program and its arguments
Returns:
this process builder

command

public List<String> command()
Returns this process builder's operating system program and arguments. The returned list is not a copy. Subsequent updates to the list will be reflected in the state of this process builder.
Returns:
this process builder's program and its arguments

environment

public Map<String,String> environment()
Returns a string map view of this process builder's environment. Whenever a process builder is created, the environment is initialized to a copy of the current process environment (see System.getenv()). Subprocesses subsequently started by this object's start() method will use this map as their environment.

The returned object may be modified using ordinary Map operations. These modifications will be visible to subprocesses started via the start() method. Two ProcessBuilder instances always contain independent process environments, so changes to the returned map will never be reflected in any other ProcessBuilder instance or the values returned by System.getenv.

If the system does not support environment variables, an empty map is returned.

The returned map does not permit null keys or values. Attempting to insert or query the presence of a null key or value will throw a NullPointerException. Attempting to query the presence of a key or value which is not of type String will throw a ClassCastException.

The behavior of the returned map is system-dependent. A system may not allow modifications to environment variables or may forbid certain variable names or values. For this reason, attempts to modify the map may fail with UnsupportedOperationException or IllegalArgumentException if the modification is not permitted by the operating system.

Since the external format of environment variable names and values is system-dependent, there may not be a one-to-one mapping between them and Java's Unicode strings. Nevertheless, the map is implemented in such a way that environment variables which are not modified by Java code will have an unmodified native representation in the subprocess.

The returned map and its collection views may not obey the general contract of the Object.equals(java.lang.Object) and Object.hashCode() methods.

The returned map is typically case-sensitive on all platforms.

If a security manager exists, its checkPermission method is called with a RuntimePermission("getenv.*") permission. This may result in a SecurityException being thrown.

When passing information to a Java subprocess, system properties are generally preferred over environment variables.

Returns:
this process builder's environment
Throws:
SecurityException - if a security manager exists and its checkPermission method doesn't allow access to the process environment
See Also:

directory

public File directory()
Returns this process builder's working directory. Subprocesses subsequently started by this object's start() method will use this as their working directory. The returned value may be null -- this means to use the working directory of the current Java process, usually the directory named by the system property user.dir, as the working directory of the child process.
Returns:
this process builder's working directory

directory

public ProcessBuilder directory(File directory)
Sets this process builder's working directory. Subprocesses subsequently started by this object's start() method will use this as their working directory. The argument may be null -- this means to use the working directory of the current Java process, usually the directory named by the system property user.dir, as the working directory of the child process.
Parameters:
directory - the new working directory
Returns:
this process builder

redirectInput

public ProcessBuilder redirectInput(ProcessBuilder.Redirect source)
Sets this process builder's standard input source. Subprocesses subsequently started by this object's start() method obtain their standard input from this source.

If the source is Redirect.PIPE (the initial value), then the standard input of a subprocess can be written to using the output stream returned by Process.getOutputStream(). If the source is set to any other value, then Process.getOutputStream() will return a null output stream.

Parameters:
source - the new standard input source
Returns:
this process builder
Throws:
IllegalArgumentException - if the redirect does not correspond to a valid source of data, that is, has type WRITE or APPEND
Since:
1.7

redirectOutput

public ProcessBuilder redirectOutput(ProcessBuilder.Redirect destination)
Sets this process builder's standard output destination. Subprocesses subsequently started by this object's start() method send their standard output to this destination.

If the destination is Redirect.PIPE (the initial value), then the standard output of a subprocess can be read using the input stream returned by Process.getInputStream(). If the destination is set to any other value, then Process.getInputStream() will return a null input stream.

Parameters:
destination - the new standard output destination
Returns:
this process builder
Throws:
IllegalArgumentException - if the redirect does not correspond to a valid destination of data, that is, has type READ
Since:
1.7

redirectError

public ProcessBuilder redirectError(ProcessBuilder.Redirect destination)
Sets this process builder's standard error destination. Subprocesses subsequently started by this object's start() method send their standard error to this destination.

If the destination is Redirect.PIPE (the initial value), then the error output of a subprocess can be read using the input stream returned by Process.getErrorStream(). If the destination is set to any other value, then Process.getErrorStream() will return a null input stream.

If the redirectErrorStream attribute has been set true, then the redirection set by this method has no effect.

Parameters:
destination - the new standard error destination
Returns:
this process builder
Throws:
IllegalArgumentException - if the redirect does not correspond to a valid destination of data, that is, has type READ
Since:
1.7

redirectInput

public ProcessBuilder redirectInput(File file)
Sets this process builder's standard input source to a file.

This is a convenience method. An invocation of the form redirectInput(file) behaves in exactly the same way as the invocation redirectInput (Redirect.from(file)).

Parameters:
file - the new standard input source
Returns:
this process builder
Since:
1.7

redirectOutput

public ProcessBuilder redirectOutput(File file)
Sets this process builder's standard output destination to a file.

This is a convenience method. An invocation of the form redirectOutput(file) behaves in exactly the same way as the invocation redirectOutput (Redirect.to(file)).

Parameters:
file - the new standard output destination
Returns:
this process builder
Since:
1.7

redirectError

public ProcessBuilder redirectError(File file)
Sets this process builder's standard error destination to a file.

This is a convenience method. An invocation of the form redirectError(file) behaves in exactly the same way as the invocation redirectError (Redirect.to(file)).

Parameters:
file - the new standard error destination
Returns:
this process builder
Since:
1.7

redirectInput

public ProcessBuilder.Redirect redirectInput()
Returns this process builder's standard input source. Subprocesses subsequently started by this object's start() method obtain their standard input from this source. The initial value is Redirect.PIPE.
Returns:
this process builder's standard input source
Since:
1.7

redirectOutput

public ProcessBuilder.Redirect redirectOutput()
Returns this process builder's standard output destination. Subprocesses subsequently started by this object's start() method redirect their standard output to this destination. The initial value is Redirect.PIPE.
Returns:
this process builder's standard output destination
Since:
1.7

redirectError

public ProcessBuilder.Redirect redirectError()
Returns this process builder's standard error destination. Subprocesses subsequently started by this object's start() method redirect their standard error to this destination. The initial value is Redirect.PIPE.
Returns:
this process builder's standard error destination
Since:
1.7

inheritIO

public ProcessBuilder inheritIO()
Sets the source and destination for subprocess standard I/O to be the same as those of the current Java process.

This is a convenience method. An invocation of the form 

 
 pb.inheritIO()
behaves in exactly the same way as the invocation 
 
 pb.redirectInput(Redirect.INHERIT)
   .redirectOutput(Redirect.INHERIT)
   .redirectError(Redirect.INHERIT)
This gives behavior equivalent to most operating system command interpreters, or the standard C library function system().
Returns:
this process builder
Since:
1.7

redirectErrorStream

public boolean redirectErrorStream()
Tells whether this process builder merges standard error and standard output.

If this property is true, then any error output generated by subprocesses subsequently started by this object's start() method will be merged with the standard output, so that both can be read using the Process.getInputStream() method. This makes it easier to correlate error messages with the corresponding output. The initial value is false.

Returns:
this process builder's redirectErrorStream property

redirectErrorStream

public ProcessBuilder redirectErrorStream(boolean redirectErrorStream)
Sets this process builder's redirectErrorStream property.

If this property is true, then any error output generated by subprocesses subsequently started by this object's start() method will be merged with the standard output, so that both can be read using the Process.getInputStream() method. This makes it easier to correlate error messages with the corresponding output. The initial value is false.

Parameters:
redirectErrorStream - the new property value
Returns:
this process builder

start

public Process start() throws IOException
Starts a new process using the attributes of this process builder.

The new process will invoke the command and arguments given by command(), in a working directory as given by directory(), with a process environment as given by environment().

This method checks that the command is a valid operating system command. Which commands are valid is system-dependent, but at the very least the command must be a non-empty list of non-null strings.

A minimal set of system dependent environment variables may be required to start a process on some operating systems. As a result, the subprocess may inherit additional environment variable settings beyond those in the process builder's environment(). The minimal set of system dependent environment variables may override the values provided in the environment.

If there is a security manager, its checkExec method is called with the first component of this object's command array as its argument. This may result in a SecurityException being thrown.

Starting an operating system process is highly system-dependent. Among the many things that can go wrong are:

  • The operating system program file was not found.
  • Access to the program file was denied.
  • The working directory does not exist.
  • Invalid character in command argument, such as NUL.

In such cases an exception will be thrown. The exact nature of the exception is system-dependent, but it will always be a subclass of IOException.

If the operating system does not support the creation of processes, an UnsupportedOperationException will be thrown.

Subsequent modifications to this process builder will not affect the returned Process.

Implementation Note:
In the reference implementation, logging of the command, arguments, directory, stack trace, and process id can be enabled. The logged information may contain sensitive security information and the potential exposure of the information should be carefully reviewed. Logging of the information is enabled when the logging level of the system logger named java.lang.ProcessBuilder is Level.DEBUG or Level.TRACE. When enabled for Level.DEBUG only the process id, directory, command, and stack trace are logged. When enabled for Level.TRACE the arguments are included with the process id, directory, command, and stack trace.
Returns:
a new Process object for managing the subprocess
Throws:
NullPointerException - if an element of the command list is null
IndexOutOfBoundsException - if the command is an empty list (has size 0)
SecurityException - if a security manager exists and
  • its checkExec method doesn't allow creation of the subprocess, or
  • the standard input to the subprocess was redirected from a file and the security manager's checkRead method denies read access to the file, or
  • the standard output or standard error of the subprocess was redirected to a file and the security manager's checkWrite method denies write access to the file
UnsupportedOperationException - If the operating system does not support the creation of processes.
IOException - if an I/O error occurs
See Also:

startPipeline

public static List<Process> startPipeline(List<ProcessBuilder> builders) throws IOException
Starts a Process for each ProcessBuilder, creating a pipeline of processes linked by their standard output and standard input streams. The attributes of each ProcessBuilder are used to start the respective process except that as each process is started, its standard output is directed to the standard input of the next. The redirects for standard input of the first process and standard output of the last process are initialized using the redirect settings of the respective ProcessBuilder. All other ProcessBuilder redirects should be Redirect.PIPE.

All input and output streams between the intermediate processes are not accessible. The standard input of all processes except the first process are null output streams The standard output of all processes except the last process are null input streams.

The redirectErrorStream() of each ProcessBuilder applies to the respective process. If set to true, the error stream is written to the same stream as standard output.

If starting any of the processes throws an Exception, all processes are forcibly destroyed.

The startPipeline method performs the same checks on each ProcessBuilder as does the start() method. Each new process invokes the command and arguments given by the respective process builder's command(), in a working directory as given by its directory(), with a process environment as given by its environment().

Each process builder's command is checked to be a valid operating system command. Which commands are valid is system-dependent, but at the very least the command must be a non-empty list of non-null strings.

A minimal set of system dependent environment variables may be required to start a process on some operating systems. As a result, the subprocess may inherit additional environment variable settings beyond those in the process builder's environment(). The minimal set of system dependent environment variables may override the values provided in the environment.

If there is a security manager, its checkExec method is called with the first component of each process builder's command array as its argument. This may result in a SecurityException being thrown.

Starting an operating system process is highly system-dependent. Among the many things that can go wrong are:

  • The operating system program file was not found.
  • Access to the program file was denied.
  • The working directory does not exist.
  • Invalid character in command argument, such as NUL.

In such cases an exception will be thrown. The exact nature of the exception is system-dependent, but it will always be a subclass of IOException.

If the operating system does not support the creation of processes, an UnsupportedOperationException will be thrown.

Subsequent modifications to any of the specified builders will not affect the returned Process.

API Note:
For example to count the unique imports for all the files in a file hierarchy on a Unix compatible platform: 

 String directory = "/home/duke/src";
 ProcessBuilder[] builders = {
              new ProcessBuilder("find", directory, "-type", "f"),
              new ProcessBuilder("xargs", "grep", "-h", "^import "),
              new ProcessBuilder("awk", "{print $2;}"),
              new ProcessBuilder("sort", "-u")};
 List<Process> processes = ProcessBuilder.startPipeline(
         Arrays.asList(builders));
 Process last = processes.get(processes.size()-1);
 try (InputStream is = last.getInputStream();
         Reader isr = new InputStreamReader(is);
         BufferedReader r = new BufferedReader(isr)) {
     long count = r.lines().count();
 }
Implementation Note:
In the reference implementation, logging of each process created can be enabled, see start() for details.
Parameters:
builders - a List of ProcessBuilders
Returns:
a List<Process>es started from the corresponding ProcessBuilder
Throws:
IllegalArgumentException - any of the redirects except the standard input of the first builder and the standard output of the last builder are not ProcessBuilder.Redirect.PIPE.
NullPointerException - if an element of the command list is null or if an element of the ProcessBuilder list is null or the builders argument is null
IndexOutOfBoundsException - if the command is an empty list (has size 0)
SecurityException - if a security manager exists and
  • its checkExec method doesn't allow creation of the subprocess, or
  • the standard input to the subprocess was redirected from a file and the security manager's checkRead method denies read access to the file, or
  • the standard output or standard error of the subprocess was redirected to a file and the security manager's checkWrite method denies write access to the file
UnsupportedOperationException - If the operating system does not support the creation of processes
IOException - if an I/O error occurs
Since:
9

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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/ProcessBuilder.html