InvocationHandler (Java SE 22 & JDK 22)
Description: declaration: module: java.base, package: java.lang.reflect, interface: InvocationHandler
Keywords: java.lang.reflect.InvocationHandler interface

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