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 {@code invoke} * method of its invocation handler. * * @author Peter Jones * @see Proxy * @since 1.3 */ public interface InvocationHandler { /** * 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. * * @param proxy the proxy instance that the method was invoked on * * @param method the {@code Method} instance corresponding to * the interface method invoked on the proxy instance. The declaring * class of the {@code 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. * * @param args an array of objects containing the values of the * arguments passed in the method invocation on the proxy instance, * or {@code null} if interface method takes no arguments. * Arguments of primitive types are wrapped in instances of the * appropriate primitive wrapper class, such as * {@code java.lang.Integer} or {@code java.lang.Boolean}. * * @return 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 * {@code null} and the interface method's return type is * primitive, then a {@code 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 {@code 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 * {@code throws} clause of the interface method or to the * unchecked exception types {@code java.lang.RuntimeException} * or {@code 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 {@code throws} clause of * the interface method, then an * {@link UndeclaredThrowableException} containing the * exception that was thrown by this method will be thrown by the * method invocation on the proxy instance. * * @see UndeclaredThrowableException */ public Object invoke(Object proxy, Method method, Object[] args) throws Throwable; /** * Invokes the specified default method on the given {@code proxy} instance with * the given parameters. The given {@code method} must be a default method * declared in a proxy interface of the {@code proxy}'s class or inherited * from its superinterface directly or indirectly. *
* Invoking this method behaves as if {@code invokespecial} instruction executed * from the proxy class, targeting the default method in a proxy interface. * This is equivalent to the invocation: * {@code X.super.m(A* a)} where {@code X} is a proxy interface and the call to * {@code X.super::m(A*)} is resolved to the given {@code method}. *
* Examples: interface {@code A} and {@code B} both declare a default * implementation of method {@code m}. Interface {@code C} extends {@code A} * and inherits the default method {@code m} from its superinterface {@code A}. * *
{@code
* interface A {
* default T m(A a) { return t1; }
* }
* interface B {
* default T m(A a) { return t2; }
* }
* interface C extends A {}
* }{@code
* 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);
* }
* });
* }{@code
* 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);
* }
* });
* }{@code
* 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);
* }
* });
* }* Adding a default method, or changing a method from abstract to default * may cause an exception if an existing code attempts to call {@code invokeDefault} * to invoke a default method. * * For example, if {@code C} is modified to implement a default method * {@code m}: * *
{@code
* interface C extends A {
* default T m(A a) { return t3; }
* }
* }* The following is another example that creates a proxy instance of {@code C} * and the invocation handler calls the {@code invokeDefault} method * to invoke {@code A::m}: * *
{@code
* 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(...);
* }