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(), new Class[] { Foo.class }); Foo f = (Foo) proxyClass. getConstructor(new Class[] { InvocationHandler.class }). newInstance(new Object[] { 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 classes are public, final, and not abstract.
- 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:proxy instanceof Foo
ClassCastException
):(Foo) proxy
- 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.
See Also
Field Summary
protected InvocationHandler | h | the invocation handler for this proxy instance. |
Protected Constructor Summary
Proxy(InvocationHandler h)
Constructs a new
Proxy instance from a subclass
(typically, a dynamic proxy class) with the specified value
for its invocation handler. |
Public Method Summary
boolean | |
static InvocationHandler | |
static Class<?> |
getProxyClass(ClassLoader loader, Class...<?> interfaces)
Returns the
java.lang.Class object for a proxy class
given a class loader and an array of interfaces. |
int |
hashCode()
Returns an integer hash code for this object.
|
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. |
static Object |
newProxyInstance(ClassLoader loader, Class[]<?> interfaces, InvocationHandler h)
Returns an instance of a proxy class for the specified interfaces
that dispatches method invocations to the specified invocation
handler.
|
String |
toString()
Returns a string containing a concise, human-readable description of this
object.
|
Inherited Method Summary
Fields
Protected Constructors
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.
Parameters
h | the invocation handler for this proxy instance |
---|
Public Methods
public boolean equals (Object obj)
Compares this instance with the specified object and indicates if they
are equal. In order to be equal, o
must represent the same object
as this instance using a class-specific comparison. The general contract
is that this comparison should be reflexive, symmetric, and transitive.
Also, no object reference other than null is equal to null.
The default implementation returns true
only if this ==
o
. See Writing a correct
equals
method
if you intend implementing your own equals
method.
The general contract for the equals
and hashCode()
methods is that if equals
returns true
for
any two objects, then hashCode()
must return the same value for
these objects. This means that subclasses of Object
usually
override either both methods or neither of them.
Parameters
obj | the object to compare this instance with. |
---|
Returns
true
if the specified object is equal to thisObject
;false
otherwise.
public static InvocationHandler getInvocationHandler (Object proxy)
Returns the invocation handler for the specified proxy instance.
Parameters
proxy | the proxy instance to return the invocation handler for |
---|
Returns
- the invocation handler for the proxy instance
Throws
IllegalArgumentException | if the argument is not a proxy instance |
---|
public static Class<?> getProxyClass (ClassLoader loader, Class...<?> interfaces)
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 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 | the class loader to define the proxy class |
---|---|
interfaces | the list of interfaces for the proxy class to implement |
Returns
- a proxy class that is defined in the specified class loader and that implements the specified interfaces
Throws
IllegalArgumentException | if any of the restrictions on the
parameters that may be passed to getProxyClass
are violated |
---|---|
NullPointerException | if the interfaces array
argument or any of its elements are null
|
public int hashCode ()
Returns an integer hash code for this object. By contract, any two
objects for which equals(Object)
returns true
must return
the same hash code value. This means that subclasses of Object
usually override both methods or neither method.
Note that hash values must not change over time unless information used in equals comparisons also changes.
See Writing a correct
hashCode
method
if you intend implementing your own hashCode
method.
Returns
- this object's hash code.
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
.
Parameters
cl | the class to test |
---|
Returns
true
if the class is a proxy class andfalse
otherwise
Throws
NullPointerException | if cl is null
|
---|
public static Object newProxyInstance (ClassLoader loader, Class[]<?> interfaces, InvocationHandler h)
Returns an instance of a proxy class for the specified interfaces that dispatches method invocations to the specified invocation handler. This method is equivalent to:
Proxy.getProxyClass(loader, interfaces). getConstructor(new Class[] { InvocationHandler.class }). newInstance(new Object[] { handler });
Proxy.newProxyInstance
throws
IllegalArgumentException
for the same reasons that
Proxy.getProxyClass
does.
Parameters
loader | the class loader to define the proxy class |
---|---|
interfaces | the list of interfaces for the proxy class to implement |
h | the invocation handler to dispatch method invocations to |
Returns
- 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
Throws
IllegalArgumentException | if any of the restrictions on the
parameters that may be passed to getProxyClass
are violated |
---|---|
NullPointerException | if the interfaces array
argument or any of its elements are null , or
if the invocation handler, h , is
null
|
public String toString ()
Returns a string containing a concise, human-readable description of this object. Subclasses are encouraged to override this method and provide an implementation that takes into account the object's type and data. The default implementation is equivalent to the following expression:
getClass().getName() + '@' + Integer.toHexString(hashCode())
See Writing a useful
toString
method
if you intend implementing your own toString
method.
Returns
- a printable representation of this object.