InvocationHandler (GNU Classpath 0.94-pre Documentation)

This interface defines an invocation handler. Suppose you are using reflection, and found a method that requires that its parameter be an object of a given interface. You want to call this method, but have no idea what classes implement that interface. So, you can create a {@link Proxy} instance, a convenient way to dynamically generate a class that meets all the necessary properties of that interface. But in order for the proxy instance to do any good, it needs to know what to do when interface methods are invoked! So, this interface is basically a cool wrapper that provides runtime code generation needed by proxy instances.

While this interface was designed for use by Proxy, it will also work on any object in general.

Hints for implementing this class:

Don't forget that Object.equals, Object.hashCode, and Object.toString will call this handler. In particular, a naive call to proxy.equals, proxy.hashCode, or proxy.toString will put you in an infinite loop. And remember that string concatenation also invokes toString.
Obey the contract of the Method object you are handling, or the proxy instance will be forced to throw a {@link NullPointerException}, {@link ClassCastException}, or {@link UndeclaredThrowableException}.
Be prepared to wrap/unwrap primitives as necessary.
The Method object may be owned by a different interface than what was actually used as the qualifying type of the method invocation in the Java source code. This means that it might not always be safe to throw an exception listed as belonging to the method's throws clause.

For a fun time, create an InvocationHandler that handles the methods of a proxy instance of the InvocationHandler interface!

Since: 1.3

See Also: Proxy

UNKNOWN: updated to 1.4

Method Summary
Object	invoke(Object proxy, Method method, Object[] args) When a method is invoked on a proxy instance, it is wrapped and this method is called instead, so that you may decide at runtime how the original method should behave.

When a method is invoked on a proxy instance, it is wrapped and this method is called instead, so that you may decide at runtime how the original method should behave.

Parameters: proxy the instance that the wrapped method should be invoked on. When this method is called by a Proxy object, `proxy' will be an instance of {@link Proxy}, and oddly enough, Proxy.getInvocationHandler(proxy) will return this! method the reflected method to invoke on the proxy. When this method is called by a Proxy object, 'method' will be the reflection object owned by the declaring class or interface, which may be a supertype of the interfaces the proxy directly implements. args the arguments passed to the original method, or null if the method takes no arguments. (But also be prepared to handle a 0-length array). Arguments of primitive type, such as boolean or int, are wrapped in the appropriate class such as {@link Boolean} or {@link Integer}.

Returns: whatever is necessary to return from the wrapped method. If the wrapped method is void, the proxy instance will ignore it. If the wrapped method returns a primitive, this must be the correct wrapper type whose value is exactly assignable to the appropriate type (no widening will be performed); a null object in this case causes a {@link NullPointerException}. In all remaining cases, if the returned object is not assignment compatible to the declared type of the original method, the proxy instance will generate a {@link ClassCastException}.

Throws: Throwable this interface is listed as throwing anything, but the implementation should only throw unchecked exceptions and exceptions listed in the throws clause of all methods being overridden by the proxy instance. If something is thrown that is not compatible with the throws clause of all overridden methods, the proxy instance will wrap the exception in an UndeclaredThrowableException. Note that an exception listed in the throws clause of the `method' parameter might not be declared in additional interfaces also implemented by the proxy object.

Interface InvocationHandler

Method Detail

invoke