Serializable
public class Proxy extends Object implements Serializable
Proxy
provides static methods for creating objects that act like instances of interfaces but allow for customized method invocation. To create a proxy instance for some interface Foo
:
InvocationHandler handler = new MyInvocationHandler(...);
Foo f = (Foo) Proxy.newProxyInstance(Foo.class.getClassLoader(),
new Class<?>[] { Foo.class },
handler);
A proxy class is a class created at runtime that implements a specified list of interfaces, known as proxy interfaces. A proxy instance is an instance of a proxy class. Each proxy instance has an associated invocation handler object, which implements the interface InvocationHandler
. A method invocation on a proxy instance through one of its proxy interfaces will be dispatched to the invoke
method of the instance's invocation handler, passing the proxy instance, a java.lang.reflect.Method
object identifying the method that was invoked, and an array of type Object
containing the arguments. The invocation handler processes the encoded method invocation as appropriate and the result that it returns will be returned as the result of the method invocation on the proxy instance.
A proxy class has the following properties:
"$Proxy"
should be, however, reserved for proxy classes. java.lang.reflect.Proxy
. getInterfaces
on its Class
object will return an array containing the same list of interfaces (in the order specified at its creation), invoking getMethods
on its Class
object will return an array of Method
objects that include all of the methods in those interfaces, and invoking getMethod
will find methods in the proxy interfaces as would be expected. ProtectionDomain
of a proxy class is the same as that of system classes loaded by the bootstrap class loader, such as java.lang.Object
, because the code for a proxy class is generated by trusted system code. This protection domain will typically be granted java.security.AllPermission
. Proxy.isProxyClass
method can be used to determine if a given class is a proxy class. A proxy instance has the following properties:
proxy
and one of the interfaces, Foo
, implemented by its proxy class, the following expression will return true:
proxy instanceof Foo
and the following cast operation will succeed (rather than throwing a ClassCastException
):
(Foo) proxy
Proxy.getInvocationHandler
method will return the invocation handler associated with the proxy instance passed as its argument. invoke
method as described in the documentation for that method. InvocationHandler::invokeDefault
. hashCode
, equals
, or toString
methods declared in java.lang.Object
on a proxy instance will be encoded and dispatched to the invocation handler's invoke
method in the same manner as interface method invocations are encoded and dispatched, as described above. The declaring class of the Method
object passed to invoke
will be java.lang.Object
. Other public methods of a proxy instance inherited from java.lang.Object
are not overridden by a proxy class, so invocations of those methods behave like they do for instances of java.lang.Object
. getProxyClass(ClassLoader, Class[])
or newProxyInstance(ClassLoader, Class[], InvocationHandler)
methods is specified as follows: Note that if proxy interfaces with a mix of accessibilities -- for example, an exported public interface and a non-exported non-public interface -- are proxied by the same instance, then the proxy class's accessibility is governed by the least accessible proxy interface.
Note that it is possible for arbitrary code to obtain access to a proxy class in an open package with setAccessible
, whereas a proxy class in a non-open package is never accessible to code outside the module of the proxy class.
Throughout this specification, a "non-exported package" refers to a package that is not exported to all modules, and a "non-open package" refers to a package that is not open to all modules. Specifically, these terms refer to a package that either is not exported/open by its containing module or is exported/open in a qualified fashion by its containing module.
A dynamic module is a named module generated at runtime. A proxy class defined in a dynamic module is encapsulated and not accessible to any module. Calling Constructor.newInstance(Object...)
on a proxy class in a dynamic module will throw IllegalAccessException
; Proxy.newProxyInstance
method should be used instead.
A dynamic module can read the modules of all of the superinterfaces of a proxy class and the modules of the classes and interfaces referenced by all public method signatures of a proxy class. If a superinterface or a referenced class or interface, say T
, is in a non-exported package, the module of T
is updated to export the package of T
to the dynamic module.
When two or more proxy interfaces contain a method with the same name and parameter signature, the order of the proxy class's interfaces becomes significant. When such a duplicate method is invoked on a proxy instance, the Method
object passed to the invocation handler will not necessarily be the one whose declaring class is assignable from the reference type of the interface that the proxy's method was invoked through. This limitation exists because the corresponding method implementation in the generated proxy class cannot determine which interface it was invoked through. Therefore, when a duplicate method is invoked on a proxy instance, the Method
object for the method in the foremost interface that contains the method (either directly or inherited through a superinterface) in the proxy class's list of interfaces is passed to the invocation handler's invoke
method, regardless of the reference type through which the method invocation occurred.
If a proxy interface contains a method with the same name and parameter signature as the hashCode
, equals
, or toString
methods of java.lang.Object
, when such a method is invoked on a proxy instance, the Method
object passed to the invocation handler will have java.lang.Object
as its declaring class. In other words, the public, non-final methods of java.lang.Object
logically precede all of the proxy interfaces for the determination of which Method
object to pass to the invocation handler.
Note also that when a duplicate method is dispatched to an invocation handler, the invoke
method may only throw checked exception types that are assignable to one of the exception types in the throws
clause of the method in all of the proxy interfaces that it can be invoked through. If the invoke
method throws a checked exception that is not assignable to any of the exception types declared by the method in one of the proxy interfaces that it can be invoked through, then an unchecked UndeclaredThrowableException
will be thrown by the invocation on the proxy instance. This restriction means that not all of the exception types returned by invoking getExceptionTypes
on the Method
object passed to the invoke
method can necessarily be thrown successfully by the invoke
method.
Modifier and Type | Field | Description |
---|---|---|
protected InvocationHandler |
h |
the invocation handler for this proxy instance. |
Modifier | Constructor | Description |
---|---|---|
protected |
Constructs a new Proxy instance from a subclass (typically, a dynamic proxy class) with the specified value for its invocation handler. |
Modifier and Type | Method | Description |
---|---|---|
static InvocationHandler |
getInvocationHandler |
Returns the invocation handler for the specified proxy instance. |
static Class |
getProxyClass |
Deprecated. Proxy classes generated in a named module are encapsulated and not accessible to code outside its module. |
static boolean |
isProxyClass |
Returns true if the given class is a proxy class. |
static Object |
newProxyInstance |
Returns a proxy instance for the specified interfaces that dispatches method invocations to the specified invocation handler. |
protected InvocationHandler h
protected Proxy(InvocationHandler h)
Proxy
instance from a subclass (typically, a dynamic proxy class) with the specified value for its invocation handler.h
- the invocation handler for this proxy instanceNullPointerException
- if the given invocation handler, h
, is null
.@Deprecated public static Class<?> getProxyClass(ClassLoader loader, Class<?>... interfaces) throws IllegalArgumentException
Constructor.newInstance
will throw IllegalAccessException
when it is called on an inaccessible proxy class. Use newProxyInstance(ClassLoader, Class[], InvocationHandler)
to create a proxy instance instead.java.lang.Class
object for a proxy class given a class loader and an array of interfaces. The proxy class will be defined by the specified class loader and will implement all of the supplied interfaces. If any of the given interfaces is non-public, the proxy class will be non-public. If a proxy class for the same permutation of interfaces has already been defined by the class loader, then the existing proxy class will be returned; otherwise, a proxy class for those interfaces will be generated dynamically and defined by the class loader.loader
- the class loader to define the proxy classinterfaces
- the list of interfaces for the proxy class to implementIllegalArgumentException
- if any of the restrictions on the parameters are violatedSecurityException
- if a security manager, s, is present and any of the following conditions is met: loader
is null
and the caller's class loader is not null
and the invocation of s.checkPermission
with RuntimePermission("getClassLoader")
permission denies access.intf
, the caller's class loader is not the same as or an ancestor of the class loader for intf
and invocation of s.checkPackageAccess()
denies access to intf
.NullPointerException
- if the interfaces
array argument or any of its elements are null
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)
IllegalArgumentException
will be thrown if any of the following restrictions is violated:
Class
objects in the given interfaces
array must represent non-hidden and non-sealed interfaces, not classes or primitive types. interfaces
array may refer to identical Class
objects. cl
and every interface i
, the following expression must be true: Class.forName(i.getName(), false, cl) == i
interfaces
array must not exceed 65535. Note that the order of the specified proxy interfaces is significant: two requests for a proxy class with the same combination of interfaces but in a different order will result in two distinct proxy classes.
loader
- the class loader to define the proxy classinterfaces
- the list of interfaces for the proxy class to implementh
- the invocation handler to dispatch method invocations toIllegalArgumentException
- if any of the restrictions on the parameters are violatedSecurityException
- if a security manager, s, is present and any of the following conditions is met: loader
is null
and the caller's class loader is not null
and the invocation of s.checkPermission
with RuntimePermission("getClassLoader")
permission denies access;intf
, the caller's class loader is not the same as or an ancestor of the class loader for intf
and invocation of s.checkPackageAccess()
denies access to intf
;s.checkPermission
with ReflectPermission("newProxyInPackage.{package name}")
permission denies access.NullPointerException
- if the interfaces
array argument or any of its elements are null
, or if the invocation handler, h
, is null
public static boolean isProxyClass(Class<?> cl)
Proxy
.cl
- the class to testtrue
if the class is a proxy class and false
otherwiseNullPointerException
- if cl
is null
public static InvocationHandler getInvocationHandler(Object proxy) throws IllegalArgumentException
proxy
- the proxy instance to return the invocation handler forIllegalArgumentException
- if the argument is not a proxy instanceSecurityException
- if a security manager, s, is present and the caller's class loader is not the same as or an ancestor of the class loader for the invocation handler and invocation of s.checkPackageAccess()
denies access to the invocation handler's class.
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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.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/reflect/Proxy.html