Package javassist.util.proxy

Class ProxyFactory

java.lang.Object
javassist.util.proxy.ProxyFactory
public class ProxyFactory extends Object
Factory of dynamic proxy classes.

This factory generates a class that extends the given super class and implements the given interfaces. The calls of the methods inherited from the super class are forwarded and then invoke() is called on the method handler associated with instances of the generated class. The calls of the methods from the interfaces are also forwarded to the method handler.

For example, if the following code is executed, 

 ProxyFactory f = new ProxyFactory();
 f.setSuperclass(Foo.class);
 f.setFilter(new MethodFilter() {
     public boolean isHandled(Method m) {
         // ignore finalize()
         return !m.getName().equals("finalize");
     }
 });
 Class c = f.createClass();
 MethodHandler mi = new MethodHandler() {
     public Object invoke(Object self, Method m, Method proceed,
                          Object[] args) throws Throwable {
         System.out.println("Name: " + m.getName());
         return proceed.invoke(self, args);  // execute the original method.
     }
 };
 Foo foo = (Foo)c.newInstance();
 ((Proxy)foo).setHandler(mi);

Here, Method is java.lang.reflect.Method.

Then, the following method call will be forwarded to MethodHandler mi and prints a message before executing the originally called method bar() in Foo. 

 foo.bar();

The last three lines of the code shown above can be replaced with a call to the helper method create, which generates a proxy class, instantiates it, and sets the method handler of the instance: 

     :
 Foo foo = (Foo)f.create(new Class[0], new Object[0], mi);

To change the method handler during runtime, execute the following code: 

 MethodHandler mi = ... ;    // alternative handler
 ((Proxy)foo).setHandler(mi);

If setHandler is never called for a proxy instance then it will employ the default handler which proceeds by invoking the original method. The behaviour of the default handler is identical to the following handler: 

 class EmptyHandler implements MethodHandler {
     public Object invoke(Object self, Method m,
                          Method proceed, Object[] args) throws Exception {
         return proceed.invoke(self, args);
     }
 }

A proxy factory caches and reuses proxy classes by default. It is possible to reset this default globally by setting static field useCache to false. Caching may also be configured for a specific factory by calling instance method setUseCache(boolean). It is strongly recommended that new clients of class ProxyFactory enable caching. Failure to do so may lead to exhaustion of the heap memory area used to store classes.

Caching is automatically disabled for any given proxy factory if deprecated instance method setHandler(MethodHandler) is called. This method was used to specify a default handler which newly created proxy classes should install when they create their instances. It is only retained to provide backward compatibility with previous releases of javassist. Unfortunately,this legacy behaviour makes caching and reuse of proxy classes impossible. The current programming model expects javassist clients to set the handler of a proxy instance explicitly by calling method Proxy.setHandler(MethodHandler) as shown in the sample code above. New clients are strongly recommended to use this model rather than calling setHandler(MethodHandler).

A proxy object generated by ProxyFactory is serializable if its super class or any of its interfaces implement java.io.Serializable. However, a serialized proxy object may not be compatible with future releases. The serialization support should be used for short-term storage or RMI.

For compatibility with older releases serialization of proxy objects is implemented by adding a writeReplace method to the proxy class. This allows a proxy to be serialized to a conventional ObjectOutputStream and deserialized from a corresponding ObjectInputStream. However this method suffers from several problems, the most notable one being that it fails to serialize state inherited from the proxy's superclass.

An alternative method of serializing proxy objects is available which fixes these problems. It requires inhibiting generation of the writeReplace method and instead using instances of ProxyObjectOutputStream and ProxyObjectInputStream (which are subclasses of ObjectOutputStream and ObjectInputStream) to serialize and deserialize, respectively, the proxy. These streams recognise javassist proxies and ensure that they are serialized and deserialized without the need for the proxy class to implement special methods such as writeReplace. Generation of the writeReplace method can be disabled globally by setting static field useWriteReplace to false. Alternatively, it may be configured per factory by calling instance method setUseWriteReplace(boolean).

Since:
3.1
Author:
Muga Nishizawa, Shigeru Chiba, Andrew Dinn
See Also:

  • Field Details

    • onlyPublicMethods

      public static boolean onlyPublicMethods

      If true, only public/protected methods are forwarded to a proxy object. The class for that proxy object is loaded by the defineClass method in java.lang.invoke.MethodHandles.Lookup, which is available in Java 9 or later. This works even when sun.misc.Unsafe is not available for some reasons (it is already deprecated in Java 9).

      To load a class, Javassist first tries to use sun.misc.Unsafe and, if not available, it uses a protected method in java.lang.ClassLoader via PrivilegedAction. Since the latter approach is not available any longer by default in Java 9 or later, the JVM argument --add-opens java.base/java.lang=ALL-UNNAMED must be given to the JVM when it is used (because of lack of sun.misc.Unsafe). If this argument cannot be given to the JVM, onlyPublicMethods should be set to true. Javassist will try to load by using java.lang.invoke.MethodHandles.Lookup.

      The default value is false.

      Since:
      3.22
      See Also:

    • writeDirectory

      public String writeDirectory
      If the value of this variable is not null, the class file of the generated proxy class is written under the directory specified by this variable. For example, if the value is ".", then the class file is written under the current directory. This method is for debugging.

      The default value is null.

    • useCache

      public static volatile boolean useCache
      If true, a generated proxy class is cached and it will be reused when generating the proxy class with the same properties is requested. The default value is true. Note that this value merely specifies the initial setting employed by any newly created proxy factory. The factory setting may be overwritten by calling factory instance method setUseCache(boolean)
      Since:
      3.4

    • useWriteReplace

      public static volatile boolean useWriteReplace
      If true, a generated proxy class will implement method writeReplace enabling serialization of its proxies to a conventional ObjectOutputStream. this (default) setting retains the old javassist behaviour which has the advantage that it retains compatibility with older releases and requires no extra work on the part of the client performing the serialization. However, it has the disadvantage that state inherited from the superclasses of the proxy is lost during serialization. if false then serialization/deserialization of the proxy instances will preserve all fields. However, serialization must be performed via a ProxyObjectOutputStream and deserialization must be via ProxyObjectInputStream. Any attempt to serialize proxies whose class was created with useWriteReplace set to false via a normal ObjectOutputStream will fail. Note that this value merely specifies the initial setting employed by any newly created proxy factory. The factory setting may be overwritten by calling factory instance method setUseWriteReplace(boolean)
      Since:
      3.4

    • classLoaderProvider

      public static ProxyFactory.ClassLoaderProvider classLoaderProvider
      A provider used by createClass() for obtaining a class loader. get() on this ClassLoaderProvider object is called to obtain a class loader.

      The value of this field can be updated for changing the default implementation.

      Example: 

       ProxyFactory.classLoaderProvider = new ProxyFactory.ClassLoaderProvider() {
     public ClassLoader get(ProxyFactory pf) {
         return Thread.currentThread().getContextClassLoader();
     }
 };
      Since:
      3.4

    • nameGenerator

      public static ProxyFactory.UniqueName nameGenerator
      A unique class name generator. Replacing this generator changes the algorithm to generate a unique name. The get method does not have to be a synchronized method since the access to this field is mutually exclusive and thus thread safe.

  • Constructor Details

    • ProxyFactory

      public ProxyFactory()
      Constructs a factory of proxy class.

  • Method Details

    • isUseCache

      public boolean isUseCache()
      test whether this factory uses the proxy cache
      Returns:
      true if this factory uses the proxy cache otherwise false

    • setUseCache

      public void setUseCache(boolean useCache)
      configure whether this factory should use the proxy cache
      Parameters:
      useCache - true if this factory should use the proxy cache and false if it should not use the cache
      Throws:
      RuntimeException - if a default interceptor has been set for the factory

    • isUseWriteReplace

      public boolean isUseWriteReplace()
      test whether this factory installs a writeReplace method in created classes
      Returns:
      true if this factory installs a writeReplace method in created classes otherwise false

    • setUseWriteReplace

      public void setUseWriteReplace(boolean useWriteReplace)
      configure whether this factory should add a writeReplace method to created classes
      Parameters:
      useWriteReplace - true if this factory should add a writeReplace method to created classes and false if it should not add a writeReplace method

    • isProxyClass

      public static boolean isProxyClass(Class<?> cl)
      determine if a class is a javassist proxy class
      Parameters:
      cl -
      Returns:
      true if the class is a javassist proxy class otherwise false

    • setSuperclass

      public void setSuperclass(Class<?> clazz)
      Sets the super class of a proxy class.

    • getSuperclass

      public Class<?> getSuperclass()
      Obtains the super class set by setSuperclass().
      Since:
      3.4

    • setInterfaces

      public void setInterfaces(Class<?>[] ifs)
      Sets the interfaces of a proxy class.

    • getInterfaces

      public Class<?>[] getInterfaces()
      Obtains the interfaces set by setInterfaces.
      Since:
      3.4

    • setFilter

      public void setFilter(MethodFilter mf)
      Sets a filter that selects the methods that will be controlled by a handler.

    • setGenericSignature

      public void setGenericSignature(String sig)
      Sets the generic signature for the proxy class.

      For example, 

      class NullPtr<T> {
     T get() { return null; }
 }

      The following code makes a proxy for NullPtr<String>: 

      ProxyFactory factory = new ProxyFactory();
 factory.setSuperclass(NullPtr.class);
 factory.setGenericSignature("LNullPtr<Ljava/lang/String;>;");
 NullPtr<?> np = (NullPtr<?>)factory.create(null, null);
java.lang.reflect.Type[] x = ((java.lang.reflect.ParameterizedType)np.getClass().getGenericSuperclass())
                                                                     .getActualTypeArguments();
 // x[0] == String.class
      Parameters:
      sig - a generic signature.
      Since:
      3.26
      See Also:

    • createClass

      public Class<?> createClass()
      Generates a proxy class using the current filter. The module or package where a proxy class is created has to be opened to this package or the Javassist module.
      See Also:

    • createClass

      public Class<?> createClass(MethodFilter filter)
      Generates a proxy class using the supplied filter. The module or package where a proxy class is created has to be opened to this package or the Javassist module.

    • createClass

      public Class<?> createClass(MethodHandles.Lookup lookup)
      Generates a proxy class using the current filter. It loads a class file by the given java.lang.invoke.MethodHandles.Lookup object, which can be obtained by MethodHandles.lookup() called from somewhere in the package that the loaded class belongs to.
      Parameters:
      lookup - used for loading the proxy class. It needs an appropriate right to invoke defineClass for the proxy class.
      Since:
      3.24

    • createClass

      public Class<?> createClass(MethodHandles.Lookup lookup, MethodFilter filter)
      Generates a proxy class using the supplied filter.
      Parameters:
      lookup - used for loading the proxy class. It needs an appropriate right to invoke defineClass for the proxy class.
      filter - the filter.
      Since:
      3.24
      See Also:

    • getKey

      public String getKey(Class<?> superClass, Class<?>[] interfaces, byte[] signature, boolean useWriteReplace)

    • getHandler

      public static MethodHandler getHandler(Proxy p)
      Obtains the method handler of the given proxy object.
      Parameters:
      p - a proxy object.
      Returns:
      the method handler.
      Since:
      3.16

    • create

      public Object create(Class<?>[] paramTypes, Object[] args, MethodHandler mh) throws NoSuchMethodException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException
      Creates a proxy class and returns an instance of that class.
      Parameters:
      paramTypes - parameter types for a constructor.
      args - arguments passed to a constructor.
      mh - the method handler for the proxy class.
      Throws:
      NoSuchMethodException
      IllegalArgumentException
      InstantiationException
      IllegalAccessException
      InvocationTargetException
      Since:
      3.4

    • create

      public Object create(Class<?>[] paramTypes, Object[] args) throws NoSuchMethodException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException
      Creates a proxy class and returns an instance of that class.
      Parameters:
      paramTypes - parameter types for a constructor.
      args - arguments passed to a constructor.
      Throws:
      NoSuchMethodException
      IllegalArgumentException
      InstantiationException
      IllegalAccessException
      InvocationTargetException

    • setHandler

      @Deprecated public void setHandler(MethodHandler mi)
      Deprecated.
      since 3.12 use of this method is incompatible with proxy class caching. instead clients should call method Proxy.setHandler(MethodHandler) to set the handler for each newly created proxy instance. calling this method will automatically disable caching of classes created by the proxy factory.
      Sets the default invocation handler. This invocation handler is shared among all the instances of a proxy class unless another is explicitly specified.