public class Proxy extends Object implements Serializable
Proxy
provides static methods for creating dynamic proxy classes and instances, and it is also the superclass of all dynamic proxy classes created by those methods.
To create a proxy for some interface Foo
:
InvocationHandler handler = new MyInvocationHandler(...); Class<?> proxyClass = Proxy.getProxyClass(Foo.class.getClassLoader(), Foo.class); Foo f = (Foo) proxyClass.getConstructor(InvocationHandler.class). newInstance(handler);or more simply:
Foo f = (Foo) Proxy.newProxyInstance(Foo.class.getClassLoader(), new Class<?>[] { Foo.class }, handler);
A dynamic proxy class (simply referred to as a proxy class below) is a class that implements a list of interfaces specified at runtime when the class is created, with behavior as described below. A proxy interface is such an interface that is implemented by a proxy class. 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. Proxy.isProxyClass
method will return true if it is passed a proxy class-- a class returned by Proxy.getProxyClass
or the class of an object returned by Proxy.newProxyInstance
-- and false otherwise. java.security.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
. InvocationHandler
, to set the invocation handler for a proxy instance. Rather than having to use the reflection API to access the public constructor, a proxy instance can be also be created by calling the Proxy.newProxyInstance
method, which combines the actions of calling Proxy.getProxyClass
with invoking the constructor with an invocation handler. A proxy instance has the following properties:
proxy
and one of the interfaces implemented by its proxy class Foo
, the following expression will return true: proxy instanceof Fooand 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. 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
. When two or more interfaces of a proxy class 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.
InvocationHandler
, Serialized Formprotected InvocationHandler h
the invocation handler for this proxy instance.
protected Proxy(InvocationHandler h)
Constructs a new 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
.public static Class<?> getProxyClass(ClassLoader loader, Class<?>... interfaces) throws IllegalArgumentException
Returns the 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.
There are several restrictions on the parameters that may be passed to Proxy.getProxyClass
:
Class
objects in the interfaces
array must represent 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. If any of these restrictions are violated, Proxy.getProxyClass
will throw an IllegalArgumentException
. If the interfaces
array argument or any of its elements are null
, a NullPointerException
will be thrown.
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 implementIllegalArgumentException
- if any of the restrictions on the parameters that may be passed to getProxyClass
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) throws IllegalArgumentException
Returns an instance of a proxy class for the specified interfaces that dispatches method invocations to the specified invocation handler.
Proxy.newProxyInstance
throws IllegalArgumentException
for the same reasons that Proxy.getProxyClass
does.
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 that may be passed to getProxyClass
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)
Returns true if and only if the specified class was dynamically generated to be a proxy class using the getProxyClass
method or the newProxyInstance
method.
The reliability of this method is important for the ability to use it to make security decisions, so its implementation should not just test if the class in question extends 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
Returns the invocation handler for the specified proxy instance.
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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