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We can use the ProxyMetaClass to intercept methods in a class within a selected block for the current thread.

Interceptors with ProxyMetaClass

By using ProxyMetaClass, we can attach an interceptor to a class for a block of code. The Groovy-supplied Interceptor interface has three methods. The beforeInvoke() method specifies code to be executed before the intercepted method, the doInvoke() indicates whether to execute the intercepted method, and afterInvoke() executes after the intercepted method finishes, or after a false-returning doInvoke(). The result parameter passed to afterInvoke() is the result of executing the method, or what was returned from beforeInvoke() if the intercepted method wasn't executed. What afterInvoke() returns is returned from the method call in the main flow of the program.

...

Code Block
class MyCategory{
  static String categorize( String s ){ "categorized: $s" }
}

class StringInterceptor implements Interceptor{
  Object beforeInvoke(Object object, String methodName, Object[] arguments){
    if( object instanceof String )
      use(MyCategory){
        assert object.&categorize() == "categorized: $object"
      }
    null
  }
  boolean doInvoke(){ true }
  Object afterInvoke(Object object, String methodName, Object[] arguments,
                     Object result){
    if( object instanceof String )
      result= "intercepted: $result"
    result
  }
}

def useInterceptor= { Class theClass, Class theInterceptor, Closure theCode->
  def proxy= ProxyMetaClass.getInstance( theClass )
  def interceptor= theInterceptor.newInstance()
  proxy.interceptor= interceptor
  proxy.use( theCode )
}

useInterceptor( String, StringInterceptor ){
  assert new String('silver').toString() == 'intercepted: silver'

  use(MyCategory){
    assert new String('golden').categorize() ==
                                     'intercepted: categorized: golden'
  }

  Thread.start{ //no interception in spawned thread...
    use(MyCategory){
      assert new String('bronze').categorize() == 'categorized: bronze'
    }
  }
}

Unintercepted Interceptors

The special & notation for bypassing interceptors handles simple code, but for more complex code we often need our own UninterceptedInterceptor:

Code Block
abstract class UninterceptedInterceptor implements Interceptor{
  def proxy= null //we need to know the proxy...

  abstract Object doBefore( Object object, String methodName,
                            Object[] arguments )

  public Object beforeInvoke( Object object, String methodName,
                              Object[] arguments ){

    proxy.interceptor= null //...so we can turn off interception...
    def result
    try{
      result= doBefore(object, methodName, arguments) 
    }catch(Exception e){
      throw e
    }finally{
      proxy.interceptor= this //...and turn interception back on
    }
    result
  }
  abstract boolean doInvoke()

  abstract Object doAfter( Object object, String methodName, Object[] arguments,
                           Object result )

  public Object afterInvoke( Object object, String methodName,
                             Object[] arguments, Object result ){
    proxy.interceptor= null //turn off interception
    try{
      result= doAfter(object, methodName, arguments, result)
    }catch(Exception e){
      throw e
    }finally{
      proxy.interceptor= this //turn interception back on
    }
    result
  }
}

class MyInterceptor extends UninterceptedInterceptor{
  Object doBefore( Object object, String methodName, Object[] arguments ){
    null
  }
  boolean doInvoke(){ true }

  Object doAfter( Object object, String methodName,Object[] arguments,
                  Object result ){
    if( object instanceof ArrayList && methodName == 'size' ){
      result = (result as Integer) + [1,2,3,4,5,6,7,8,9,10].size()
                //call ArrayList size() method here without stack overflow
    }
    result
  }
}

def useInterceptor= { Class theClass, Class theInterceptor, Closure theCode->
  def proxy= ProxyMetaClass.getInstance( theClass )
  def interceptor= theInterceptor.newInstance()
  proxy.interceptor= interceptor
  interceptor.proxy= proxy
        //we must now store a proxy reference in the interceptor
  proxy.use( theCode )
}

useInterceptor( ArrayList, MyInterceptor ){
  assert ['a', 'b', 'c'].size() == 13
}

Intercepting many classes in one block

Often, we want to intercept more than one class in one block. This example is of an aliasing interceptor, which disables some English-language names for selected classes, and replaces them with Spanish-language names. We re-use the UninterceptedInterceptor class and useInterceptor utility from the previous example.

...

Code Block
class Extras{
  static closureInject(List self, Closure base){
    def z= []
    self.eachWithIndex{ it, i-> z<< {-> it( z[i+1] )} }
    z<< base
    z[0]()
  }
}

use(Extras){
  [ {c-> useInterceptor(ArrayList, ArrayListAliasInterceptor){ c() }},
    {c-> useInterceptor(HashMap, HashMapAliasInterceptor){ c() }},
    {c-> useInterceptor(LinkedHashMap, LinkedHashMapAliasInterceptor){ c() }},

  ].closureInject{
    def a= [1, 3, 5, 7, 9],
        b= [a:1, c:3, e:5, g:7],
        c= new LinkedHashMap( [e:5, g:7, i:9] )

    println 'size: '+ a.tamano()
    try{ println a.size(); assert 0 }
    catch(e){ assert e instanceof MissingMethodException }
    a.todos{ println 'item: '+ it }
    println ''

    println 'size: '+ b.tamano()
    try{ println b.size(); assert 0 }
    catch(e){ assert e instanceof MissingMethodException }
    b.todos{ println 'item: '+ it }
    println ''

    println 'size: '+ c.tamano()
    try{ println c.size(); assert 0 }
    catch(e){ assert e instanceof MissingMethodException }
    c.todos{ println 'item: '+ it }
  }
}

Our own ProxyMetaClass

We can define our own proxy meta-classes. One case for which we'd do so is to implement our own style of interceptors, here, an around-interceptor:

...

Code Block
class MyInterceptor implements AroundInterceptor{
  Object aroundInvoke(Object object, String methodName, Object[] arguments,
                      Closure proceed){
    println "  BEFORE $object .$methodName $arguments"
    def result= proceed()
    println "  AFTER $object .$methodName $arguments: $result"
    result
  }
}

class MyClass{
  void sayHi(){ System.out.println 'hi' }
}

def interceptor= new MyInterceptor()
def proxy= MyProxyMetaClass.getInstance( MyClass )
proxy.use{
  proxy.interceptor= interceptor
  new MyClass().sayHi()
}

/*outputs:
  BEFORE class MyClass .ctor {}
  AFTER class MyClass .ctor {}: MyClass@1f5d386
  BEFORE MyClass@1f5d386 .sayHi {}
hi
  AFTER MyClass@1f5d386 .sayHi {}: null
*/

Using many Interceptors with our own ProxyMetaClass

We can only use one interceptor with the ProxyMetaClass supplied by Groovy, so we need to provide our own when attaching more than one interceptor to a class:

Code Block
import org.codehaus.groovy.runtime.InvokerHelper

public class MultiInterceptorProxyMetaClass extends MetaClassImpl{
  protected adaptee= null
  def interceptors= [] //reference a list of interceptors, instead of just one

  MultiInterceptorProxyMetaClass( MetaClassRegistry registry, Class theClass,
                                  MetaClass adaptee ){
    super(registry, theClass)
    this.adaptee = adaptee
    if( null == adaptee )
        throw new IllegalArgumentException( "adaptee must not be null" )
  }
  static getInstance(Class theClass){
    def metaRegistry= InvokerHelper.getInstance().getMetaRegistry()
    new MultiInterceptorProxyMetaClass( metaRegistry, theClass, 
                                        metaRegistry.getMetaClass(theClass) )
  }
  void use(Closure closure){
    registry.setMetaClass(theClass, this)
    registry.getMetaClass(theClass).initialize()
    try{ closure.call() }
    finally{ registry.setMetaClass(theClass, adaptee) }
  }
  void use(GroovyObject object, Closure closure){
    object.setMetaClass(this)
    try{ closure.call() }
    finally{ object.setMetaClass(adaptee) }
  }
  Object invokeMethod( final Object object, final String methodName,
                       final Object[] arguments ){
    doCall(object, methodName, arguments,
           { adaptee.invokeMethod(object, methodName, arguments) } )
  }
  Object invokeStaticMethod( final Object object, final String methodName,
                             final Object[] arguments ){
    doCall(object, methodName, arguments,
           { adaptee.invokeStaticMethod(object, methodName, arguments) } )
  }
  Object invokeConstructor(final Object[] arguments){
    doCall(theClass, "ctor", arguments,
           { adaptee.invokeConstructor(arguments) } )
  }
  public Object invokeConstructorAt(final Class at, final Object[] arguments){
    doCall(theClass, "ctor", arguments,
           { adaptee.invokeConstructorAt(at, arguments) } )
  }
  private Object doCall( Object object, String methodName, Object[] arguments,
                         Closure howToInvoke ){
    if( interceptors == [] ){ return howToInvoke.call() }
    def result
    interceptors.each{ //different logic to cater for all the interceptors

      result= it.beforeInvoke(object, methodName, arguments)
      if( it.doInvoke() ){ result= howToInvoke.call() }
      it.afterInvoke(object, methodName, arguments, result)
    }
    result
  }
}

Using a MultiInterceptorProxyMetaClass for the Observer pattern

A common design pattern is the Observer pattern. Using interceptors, we can abstract the observation code into its own class, the ObserverProtocol, which can be used by subclasses. It enables us to add and remove observing objects for an observed object. We use method interception to decouple the observing and observed objects from the observation relationship itself.

...

Code Block
import java.awt.Color

def colorObserver= new ColorObserver()
def coordinateObserver= new CoordinateObserver()
def screenObserver= new ScreenObserver()

def pointProxy= MultiInterceptorProxyMetaClass.getInstance( Point )
pointProxy.interceptors << colorObserver << coordinateObserver
                                               //multi-interception used here
pointProxy.use{

  def screenProxy= MultiInterceptorProxyMetaClass.getInstance( Screen )
  screenProxy.interceptors << screenObserver
  screenProxy.use{

    println("Creating Screen s1,s2,s3,s4,s5 and Point p")
    def s1= new Screen('s1'),
        s2= new Screen('s2'),
        s3= new Screen('s3'),
        s4= new Screen('s4'),
        s5= new Screen('s5')
    def p= new Point(5, 5, Color.blue)

    println("Creating observing relationships:")
    println(" - s1 and s2 observe color changes to p")
    println(" - s3 and s4 observe coordinate changes to p")
    println(" - s5 observes s2's and s4's display() method")

    colorObserver.addObserver(p, s1)
    colorObserver.addObserver(p, s2)
    coordinateObserver.addObserver(p, s3)
    coordinateObserver.addObserver(p, s4)
    screenObserver.addObserver(s2, s5)
    screenObserver.addObserver(s4, s5)

    println("Changing p's color:")
    p.setColor(Color.red)

    println("Changing p's x-coordinate:")
    p.setX(4)

    println("done.")
  }
}

/*output:
Creating Screen s1,s2,s3,s4,s5 and Point p
Creating observing relationships:
- s1 and s2 observe color changes to p
- s3 and s4 observe coordinate changes to p
- s5 observes s2's and s4's display() method
Changing p's color:
s1: Screen updated (point subject changed color).
s2: Screen updated (point subject changed color).
s5: Screen updated (screen subject changed message).
Changing p's x-coordinate:
s3: Screen updated (point subject changed coordinates).
s4: Screen updated (point subject changed coordinates).
s5: Screen updated (screen subject changed message).
done.
*/

Using a MultiInterceptorProxyMetaClass and UninterceptedFriendlyInterceptor for the Decorator pattern

We can use more than one unintercepted interceptor with a proxy meta-class. A good example where this is necessary is the Decorator pattern. We re-use the MultiInterceptorProxyMetaClass from previous examples, but must write a special unintercepted interceptor, which we call an UninterceptedFriendlyInterceptor, that can be used as one of many with the MultiInterceptorProxyMetaClass.

...