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public interface InvocationHandler
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 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:
NullPointerException
, ClassCastException
,
or UndeclaredThrowableException
.For a fun time, create an InvocationHandler that handles the methods of a proxy instance of the InvocationHandler interface!
Proxy
,
UndeclaredThrowableException
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. |
Method Detail |
---|
Object invoke(Object proxy, Method method, Object[] args) throws Throwable
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 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 Boolean
or Integer
.
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
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 ClassCastException
.
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.Proxy
,
UndeclaredThrowableException
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