Proxy
open class Proxy : Serializable
kotlin.Any | |
↳ | java.lang.reflect.Proxy |
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);
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 classes are public, final, and not abstract if all proxy interfaces are public.
- Proxy classes are non-public, final, and not abstract if any of the proxy interfaces is non-public.
- The unqualified name of a proxy class is unspecified. The space of class names that begin with the string
"$Proxy"
should be, however, reserved for proxy classes. - A proxy class extends
java.lang.reflect.Proxy
. - A proxy class implements exactly the interfaces specified at its creation, in the same order.
- If a proxy class implements a non-public interface, then it will be defined in the same package as that interface. Otherwise, the package of a proxy class is also unspecified. Note that package sealing will not prevent a proxy class from being successfully defined in a particular package at runtime, and neither will classes already defined by the same class loader and the same package with particular signers.
- Since a proxy class implements all of the interfaces specified at its creation, invoking
getInterfaces
on itsClass
object will return an array containing the same list of interfaces (in the order specified at its creation), invokinggetMethods
on itsClass
object will return an array ofMethod
objects that include all of the methods in those interfaces, and invokinggetMethod
will find methods in the proxy interfaces as would be expected. - The
Proxy.isProxyClass
method will return true if it is passed a proxy class-- a class returned byProxy.getProxyClass
or the class of an object returned byProxy.newProxyInstance
-- and false otherwise. - The
java.security.ProtectionDomain
of a proxy class is the same as that of system classes loaded by the bootstrap class loader, such asjava.lang.Object
, because the code for a proxy class is generated by trusted system code. This protection domain will typically be grantedjava.security.AllPermission
. - Each proxy class has one public constructor that takes one argument, an implementation of the interface
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 theProxy.newProxyInstance
method, which combines the actions of callingProxy.getProxyClass
with invoking the constructor with an invocation handler.
A proxy instance has the following properties:
- Given a proxy instance
proxy
and one of the interfaces implemented by its proxy classFoo
, the following expression will return true: and the following cast operation will succeed (rather than throwing a<code>proxy instanceof Foo</code>
ClassCastException
):<code>(Foo) proxy</code>
- Each proxy instance has an associated invocation handler, the one that was passed to its constructor. The static
Proxy.getInvocationHandler
method will return the invocation handler associated with the proxy instance passed as its argument. - An interface method invocation on a proxy instance will be encoded and dispatched to the invocation handler's
invoke
method as described in the documentation for that method. - An invocation of the
hashCode
,equals
, ortoString
methods declared injava.lang.Object
on a proxy instance will be encoded and dispatched to the invocation handler'sinvoke
method in the same manner as interface method invocations are encoded and dispatched, as described above. The declaring class of theMethod
object passed toinvoke
will bejava.lang.Object
. Other public methods of a proxy instance inherited fromjava.lang.Object
are not overridden by a proxy class, so invocations of those methods behave like they do for instances ofjava.lang.Object
.
Methods Duplicated in Multiple Proxy Interfaces
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.
Summary
Protected constructors | |
---|---|
Constructs a new |
Public methods | |
---|---|
open static InvocationHandler |
getInvocationHandler(proxy: Any) Returns the invocation handler for the specified proxy instance. |
open static Class<*> |
getProxyClass(loader: ClassLoader?, vararg interfaces: Class<*>!) Returns the |
open static Boolean |
isProxyClass(cl: Class<*>) Returns true if and only if the specified class was dynamically generated to be a proxy class using the |
open static Any |
newProxyInstance(loader: ClassLoader?, interfaces: Array<Class<*>!>, h: InvocationHandler) Returns an instance of a proxy class for the specified interfaces that dispatches method invocations to the specified invocation handler. |
Properties | |
---|---|
InvocationHandler! |
the invocation handler for this proxy instance. |
Protected constructors
Proxy
protected Proxy(h: InvocationHandler)
Constructs a new Proxy
instance from a subclass (typically, a dynamic proxy class) with the specified value for its invocation handler.
Parameters | |
---|---|
h |
InvocationHandler: the invocation handler for this proxy instance |
Exceptions | |
---|---|
java.lang.NullPointerException |
if the given invocation handler, h , is null . |
Public methods
getInvocationHandler
open static fun getInvocationHandler(proxy: Any): InvocationHandler
Returns the invocation handler for the specified proxy instance.
Parameters | |
---|---|
proxy |
Any: the proxy instance to return the invocation handler for |
Return | |
---|---|
InvocationHandler |
the invocation handler for the proxy instance |
Exceptions | |
---|---|
java.lang.IllegalArgumentException |
if the argument is not a proxy instance |
java.lang.SecurityException |
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. |
getProxyClass
open static fun getProxyClass(
loader: ClassLoader?,
vararg interfaces: Class<*>!
): Class<*>
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
:
- All of the
Class
objects in theinterfaces
array must represent interfaces, not classes or primitive types. - No two elements in the
interfaces
array may refer to identicalClass
objects. - All of the interface types must be visible by name through the specified class loader. In other words, for class loader
cl
and every interfacei
, the following expression must be true:Class.forName(i.getName(), false, cl) == i
- All non-public interfaces must be in the same package; otherwise, it would not be possible for the proxy class to implement all of the interfaces, regardless of what package it is defined in.
- For any set of member methods of the specified interfaces that have the same signature:
- If the return type of any of the methods is a primitive type or void, then all of the methods must have that same return type.
- Otherwise, one of the methods must have a return type that is assignable to all of the return types of the rest of the methods.
- The resulting proxy class must not exceed any limits imposed on classes by the virtual machine. For example, the VM may limit the number of interfaces that a class may implement to 65535; in that case, the size of the
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.
Parameters | |
---|---|
loader |
ClassLoader?: the class loader to define the proxy class |
interfaces |
Class<*>!: the list of interfaces for the proxy class to implement |
Return | |
---|---|
Class<*> |
a proxy class that is defined in the specified class loader and that implements the specified interfaces |
Exceptions | |
---|---|
java.lang.IllegalArgumentException |
if any of the restrictions on the parameters that may be passed to getProxyClass are violated |
java.lang.SecurityException |
if a security manager, s, is present and any of the following conditions is met:
|
java.lang.NullPointerException |
if the interfaces array argument or any of its elements are null |
isProxyClass
open static fun isProxyClass(cl: Class<*>): Boolean
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
.
Parameters | |
---|---|
cl |
Class<*>: the class to test |
Return | |
---|---|
Boolean |
true if the class is a proxy class and false otherwise |
Exceptions | |
---|---|
java.lang.NullPointerException |
if cl is null |
newProxyInstance
open static fun newProxyInstance(
loader: ClassLoader?,
interfaces: Array<Class<*>!>,
h: InvocationHandler
): Any
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.
Parameters | |
---|---|
loader |
ClassLoader?: the class loader to define the proxy class |
interfaces |
Array<Class<*>!>: the list of interfaces for the proxy class to implement |
h |
InvocationHandler: the invocation handler to dispatch method invocations to |
Return | |
---|---|
Any |
a proxy instance with the specified invocation handler of a proxy class that is defined by the specified class loader and that implements the specified interfaces |
Exceptions | |
---|---|
java.lang.IllegalArgumentException |
if any of the restrictions on the parameters that may be passed to getProxyClass are violated |
java.lang.SecurityException |
if a security manager, s, is present and any of the following conditions is met:
|
java.lang.NullPointerException |
if the interfaces array argument or any of its elements are null , or if the invocation handler, h , is null |
Properties
h
protected var h: InvocationHandler!
the invocation handler for this proxy instance.