jdk.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);
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 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 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.
The 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.
The 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.
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 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:
Given a proxy instance proxy and one of the interfaces implemented by its proxy class Foo, the following expression will return true:
proxy instanceof Foo
and the following cast operation will succeed (rather than throwing a 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, 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.
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.
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 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 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.
The 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.
The 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.
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 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:
Given a proxy instance proxy and one of the
interfaces implemented by its proxy class Foo, the
following expression will return true:
proxy instanceof Foo
and the following cast operation will succeed (rather than throwing
a 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, 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.
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.*get-invocation-handlerclj
(*get-invocation-handler proxy)Returns the invocation handler for the specified proxy instance.
proxy - the proxy instance to return the invocation handler for - java.lang.Object
returns: the invocation handler for the proxy instance - java.lang.reflect.InvocationHandler
throws: java.lang.IllegalArgumentException - if the argument is not a proxy instance
Returns the invocation handler for the specified proxy instance. proxy - the proxy instance to return the invocation handler for - `java.lang.Object` returns: the invocation handler for the proxy instance - `java.lang.reflect.InvocationHandler` throws: java.lang.IllegalArgumentException - if the argument is not a proxy instance
*get-proxy-classclj
(*get-proxy-class loader 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 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 the interfaces array must represent interfaces, not classes or primitive types.
No two elements in the interfaces array may refer to identical Class 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 interface i, 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.
loader - the class loader to define the proxy class - java.lang.ClassLoader
interfaces - the list of interfaces for the proxy class to implement - java.lang.Class
returns: a proxy class that is defined in the specified class loader
and that implements the specified interfaces - java.lang.Class<?>
throws: java.lang.IllegalArgumentException - if any of the restrictions on the parameters that may be passed to getProxyClass are violated
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 the
interfaces array must represent interfaces, not
classes or primitive types.
No two elements in the interfaces array may
refer to identical Class 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 interface i, 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.
loader - the class loader to define the proxy class - `java.lang.ClassLoader`
interfaces - the list of interfaces for the proxy class to implement - `java.lang.Class`
returns: a proxy class that is defined in the specified class loader
and that implements the specified interfaces - `java.lang.Class<?>`
throws: java.lang.IllegalArgumentException - if any of the restrictions on the parameters that may be passed to getProxyClass are violated*new-proxy-instanceclj
(*new-proxy-instance loader interfaces h)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 class - java.lang.ClassLoader
interfaces - the list of interfaces for the proxy class to implement - java.lang.Class[]
h - the invocation handler to dispatch method invocations to - java.lang.reflect.InvocationHandler
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 - java.lang.Object
throws: java.lang.IllegalArgumentException - if any of the restrictions on the parameters that may be passed to getProxyClass are violated
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 class - `java.lang.ClassLoader`
interfaces - the list of interfaces for the proxy class to implement - `java.lang.Class[]`
h - the invocation handler to dispatch method invocations to - `java.lang.reflect.InvocationHandler`
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 - `java.lang.Object`
throws: java.lang.IllegalArgumentException - if any of the restrictions on the parameters that may be passed to getProxyClass are violated*proxy-class?clj
(*proxy-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 test - java.lang.Class
returns: true if the class is a proxy class and
false otherwise - boolean
throws: java.lang.NullPointerException - if cl is null
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 test - `java.lang.Class`
returns: true if the class is a proxy class and
false otherwise - `boolean`
throws: java.lang.NullPointerException - if cl is nullcljdoc is a website building & hosting documentation for Clojure/Script libraries
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