Module java.base

Package java.lang.reflect

Interface InvocationHandler

All Known Implementing Classes:

CompositeDataInvocationHandler, EventHandler, MBeanServerInvocationHandler, RemoteObjectInvocationHandler

public interface InvocationHandler

InvocationHandler is the interface implemented by the invocation handler of a proxy instance.

Each proxy instance has an associated invocation handler. When a method is invoked on a proxy instance, the method invocation is encoded and dispatched to the invoke method of its invocation handler.

Since:

1.3

See Also:

• Proxy

Method Summary

Modifier and Type	Method	Description
Object	invoke(Object proxy, Method method, Object[] args)	Processes a method invocation on a proxy instance and returns the result.
static Object	invokeDefault(Object proxy, Method method, Object... args)	Invokes the specified default method on the given proxy instance with the given parameters.

Method Details

invoke

Object invoke(Object proxy,
 Method method,
 Object[] args)
       throws Throwable

Processes a method invocation on a proxy instance and returns the result. This method will be invoked on an invocation handler when a method is invoked on a proxy instance that it is associated with.

Parameters:

proxy - the proxy instance that the method was invoked on

method - the Method instance corresponding to the interface method invoked on the proxy instance. The declaring class of the Method object will be the interface that the method was declared in, which may be a superinterface of the proxy interface that the proxy class inherits the method through.

args - an array of objects containing the values of the arguments passed in the method invocation on the proxy instance, or null if interface method takes no arguments. Arguments of primitive types are wrapped in instances of the appropriate primitive wrapper class, such as java.lang.Integer or java.lang.Boolean.

Returns:

the value to return from the method invocation on the proxy instance. If the declared return type of the interface method is a primitive type, then the value returned by this method must be an instance of the corresponding primitive wrapper class; otherwise, it must be a type assignable to the declared return type. If the value returned by this method is null and the interface method's return type is primitive, then a NullPointerException will be thrown by the method invocation on the proxy instance. If the value returned by this method is otherwise not compatible with the interface method's declared return type as described above, a ClassCastException will be thrown by the method invocation on the proxy instance.

Throws:

Throwable - the exception to throw from the method invocation on the proxy instance. The exception's type must be assignable either to any of the exception types declared in the throws clause of the interface method or to the unchecked exception types java.lang.RuntimeException or java.lang.Error. If a checked exception is thrown by this method that is not assignable to any of the exception types declared in the throws clause of the interface method, then an UndeclaredThrowableException containing the exception that was thrown by this method will be thrown by the method invocation on the proxy instance.

See Also:

• UndeclaredThrowableException

invokeDefault

static Object invokeDefault(Object proxy,
 Method method,
 Object... args)
                     throws Throwable

Invokes the specified default method on the given proxy instance with the given parameters. The given method must be a default method declared in a proxy interface of the proxy's class or inherited from its superinterface directly or indirectly.

Invoking this method behaves as if invokespecial instruction executed from the proxy class, targeting the default method in a proxy interface. This is equivalent to the invocation: X.super.m(A* a) where X is a proxy interface and the call to X.super::m(A*) is resolved to the given method.

Examples: interface A and B both declare a default implementation of method m. Interface C extends A and inherits the default method m from its superinterface A.

 interface A {
     default T m(A a) { return t1; }
 }
 interface B {
     default T m(A a) { return t2; }
 }
 interface C extends A {}
 

The following creates a proxy instance that implements A and invokes the default method A::m.

 Object proxy = Proxy.newProxyInstance(loader, new Class<?>[] { A.class },
         (o, m, params) -> {
             if (m.isDefault()) {
                 // if it's a default method, invoke it
                 return InvocationHandler.invokeDefault(o, m, params);
             }
         });
 

If a proxy instance implements both A and B, both of which provides the default implementation of method m, the invocation handler can dispatch the method invocation to A::m or B::m via the invokeDefault method. For example, the following code delegates the method invocation to B::m.

 Object proxy = Proxy.newProxyInstance(loader, new Class<?>[] { A.class, B.class },
         (o, m, params) -> {
             if (m.getName().equals("m")) {
                 // invoke B::m instead of A::m
                 Method bMethod = B.class.getMethod(m.getName(), m.getParameterTypes());
                 return InvocationHandler.invokeDefault(o, bMethod, params);
             }
         });
 

If a proxy instance implements C that inherits the default method m from its superinterface A, then the interface method invocation on "m" is dispatched to the invocation handler's invoke method with the Method object argument representing the default method A::m.

 Object proxy = Proxy.newProxyInstance(loader, new Class<?>[] { C.class },
        (o, m, params) -> {
             if (m.isDefault()) {
                 // behaves as if calling C.super.m(params)
                 return InvocationHandler.invokeDefault(o, m, params);
             }
        });
 

The invocation of method "m" on this proxy will behave as if C.super::m is called and that is resolved to invoking A::m.

Adding a default method, or changing a method from abstract to default may cause an exception if an existing code attempts to call invokeDefault to invoke a default method. For example, if C is modified to implement a default method m:

 interface C extends A {
     default T m(A a) { return t3; }
 }
 

The code above that creates proxy instance proxy with the modified C will run with no exception and it will result in calling C::m instead of A::m.

The following is another example that creates a proxy instance of C and the invocation handler calls the invokeDefault method to invoke A::m:

 C c = (C) Proxy.newProxyInstance(loader, new Class<?>[] { C.class },
         (o, m, params) -> {
             if (m.getName().equals("m")) {
                 // IllegalArgumentException thrown as {@code A::m} is not a method
                 // inherited from its proxy interface C
                 Method aMethod = A.class.getMethod(m.getName(), m.getParameterTypes());
                 return InvocationHandler.invokeDefault(o, aMethod params);
             }
         });
 c.m(...);
 

The above code runs successfully with the old version of C and A::m is invoked. When running with the new version of C, the above code will fail with IllegalArgumentException because C overrides the implementation of the same method and A::m is not accessible by a proxy instance.

API Note:

The proxy parameter is of type Object rather than Proxy to make it easy for InvocationHandler::invoke implementation to call directly without the need of casting.

Parameters:

proxy - the Proxy instance on which the default method to be invoked

method - the Method instance corresponding to a default method declared in a proxy interface of the proxy class or inherited from its superinterface directly or indirectly

args - the parameters used for the method invocation; can be null if the number of formal parameters required by the method is zero.

Returns:

the value returned from the method invocation

Throws:

IllegalArgumentException - if any of the following conditions is true:

• proxy is not a proxy instance; or
• the given method is not a default method declared in a proxy interface of the proxy class and not inherited from any of its superinterfaces; or
• the given method is overridden directly or indirectly by the proxy interfaces and the method reference to the named method never resolves to the given method; or
• the length of the given args array does not match the number of parameters of the method to be invoked; or
• any of the args elements fails the unboxing conversion if the corresponding method parameter type is a primitive type; or if, after possible unboxing, any of the args elements cannot be assigned to the corresponding method parameter type.

IllegalAccessException - if the declaring class of the specified default method is inaccessible to the caller class

NullPointerException - if proxy or method is null

Throwable - anything thrown by the default method

See Java Virtual Machine Specification:

5.4.3. Method Resolution

Since:

16