Java proxy get object

Understanding Java：Proxy

The Proxy is a design pattern in software design. It provides a surrogate or placeholder for another object to control access to it. It lets you call a method of a proxy object the same as calling a method of a POJO. But the proxy’s methods can do additional things than POJO’s method.

Why Do We Need Proxy

• It lets you control to access an object. For example, to delay an object’s construction and initialization.
• It lets you track the object access. For example, tracking methods access that can analyze user behavior or analyze system performance.

How to Use Proxy in Java

There are commonly three ways to implement the proxy pattern in Java: static proxy, JDK dynamic proxy, and CGLib dynamic proxy.

Static Proxy

The static proxy implemented by reference the common class in proxy class. The following example is a basic static proxy implementation:

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public interface Subject{
void request();
}

public class RealSubject implements Subject{
public void request(){
System.out.println(«request to RealSubject. «);
}
}

public interface Proxy extends Subject{}

public class ConcreteProxy implements Proxy{
private RealSubject realSubject = new RealSubject();
public void request(){
System.out.println(«before. «);
realSubject.request();
System.out.println(«after. «);
}
}

public class Client{
public static void main(String[] args){
Subject subject = new ConcreteProxy();
subject.request();
}
}

JDK Dynamic Proxy

The JDK dynamic proxy implemented in package java.lang.reflect of Java API. Its underlying implementation is the Java reflection.

The manly class for implemented dynamic proxy are: the interface java.lang.reflect.InvocationHandler , and the class java.lang.reflect.Proxy .

You can call the Proxy ‘s a static method Object newProxyInstance(ClassLoader loader, Class[] interfaces, InvocationHandler h) to get a proxy object.

• The ClassLoader instance commonly use ClassLoader.getSystemClassLoader() .
• The array of Class Class[] is the interfaces you want to implement.
• The InvocationHandler instance is an invocation handler that handling proxy method calling with the method invoke(Object proxy, Method method, Object[] args) . We need to create an invocation handler by implementing the interface InvocationHandler .

The following code example is a basic JDK Dynamic proxy implementation:

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public interface Subject{
void request();
}

public class RealSubject implements Subject{
public void request(){
System.out.println(«request to RealSubject. «);
}
}

public class MyInvocationHandler implements InvocationHandler {
private RealSubject realSubject = new RealSubject();

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println(«before. «);
Object result = method.invoke(realSubject, args);
System.out.println(«after. «);
return result;
}
}

public class Client {
public static void main(String[] args) {
Subject subject = (Subject) Proxy.newProxyInstance(
ClassLoader.getSystemClassLoader(),
new Class[]{Subject.class}, new MyInvocationHandler());
subject.request();
}
}

Properties of Proxy Classes

• All proxy classes extend the class java.lang.reflect.Proxy .
• A proxy class has only one instance field–the invocation handler, which is defined in the Proxy superclass.
• Any additional data required to carry out the proxy objects’ tasks must be stored in the invocation handler. The invocation handler wrapped the actual objects.
• The name of proxy classes are not defined. The Proxy class in Java virtual machine generates class names that begin with string $Proxy.
• There is only one proxy class for a particular class loader and ordered set of interfaces. If you call the newProxyInstance method twice with the same class loader and interface array, you get two objects of the same class. You can get class by Proxy.getProxyClass() . You can test whether a particular Class object represents a proxy class by calling the isProxyClass method of the Proxy class.

CGLib Dynamic Proxy

CGLib (Code Generation Library) is an open source library that capable creating and loading class files in memory during Java runtime. To do that it uses Java bytecode generation library ‘asm’, which is a very low-level bytecode creation tool. CGLib can proxy to objects without Interface.

dependency>
groupId>cglib groupId>
artifactId>cglib artifactId>
version>3.3.0 version>
dependency>

How to Use CGLib

To create a proxy object using CGLib is almost as simple as using the JDK reflection proxy API. The following code example is a basic CGLib Dynamic proxy implementation:

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public interface Subject{
void request();
}

public class RealSubject implements Subject{
public void request(){
System.out.println(«request to RealSubject. «);
}
}

public class MyMethodInterceptor implements MethodInterceptor {
private final Subject realSubject;

public MyMethodInterceptor(Subject subject){
this.realSubject = subject;
}

@Override
public Object intercept(Object o, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
System.out.println(«before. «);
Object result = method.invoke(realSubject, args);
System.out.println(«after. «);
return result;
}
}

public class Client {
public static void main(String[] args) {
Subject subject = new RealSubject();
MethodInterceptor methodInterceptor = new MyMethodInterceptor(subject);
Subject proxy = (Subject) Enhancer.create(Subject.class, methodInterceptor);
proxy.request();
}
}

The difference between JDK dynamic proxy and CGLib is that name of the classes are a bit different and CGLib do not have an interface.

It is also important that the proxy class extends the original class and thus when the proxy object is created it invokes the constructor of the original class.

Conclusion

Differences between JDK proxy and CGLib:

• JDK Dynamic proxy can only proxy by interface (so your target class needs to implement an interface, which is then also implemented by the proxy class). CGLib (and javassist) can create a proxy by subclassing. In this scenario the proxy becomes a subclass of the target class. No need for interfaces.
• JDK Dynamic proxy generates dynamically at runtime using JDK Reflection API. CGLib is built on top of ASM, this is mainly used the generate proxy extending bean and adds bean behavior in the proxy methods.

References

[1] Core Java Volume I Fundamentals by S, Horstmann [2] Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides

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Java proxy get object

Proxy provides static methods for creating dynamic proxy classes and instances, and it is also the superclass of all dynamic proxy classes created by those methods. To create a proxy for some interface Foo :

InvocationHandler handler = new MyInvocationHandler(. ); Class proxyClass = Proxy.getProxyClass(Foo.class.getClassLoader(), Foo.class); Foo f = (Foo) proxyClass.getConstructor(InvocationHandler.class). newInstance(handler);

Foo f = (Foo) Proxy.newProxyInstance(Foo.class.getClassLoader(), new Class[] < Foo.class >, handler);

• Proxy classes are public, final, and not abstract if all proxy interfaces are public.
• Proxy classes are non-public, final, and not abstract if any of the proxy interfaces is non-public.
• The unqualified name of a proxy class is unspecified. The space of class names that begin with the string «$Proxy» should be, however, reserved for proxy classes.
• A proxy class extends java.lang.reflect.Proxy .
• A proxy class implements exactly the interfaces specified at its creation, in the same order.
• If a proxy class implements a non-public interface, then it will be defined in the same package as that interface. Otherwise, the package of a proxy class is also unspecified. Note that package sealing will not prevent a proxy class from being successfully defined in a particular package at runtime, and neither will classes already defined by the same class loader and the same package with particular signers.
• Since a proxy class implements all of the interfaces specified at its creation, invoking getInterfaces on its Class object will return an array containing the same list of interfaces (in the order specified at its creation), invoking getMethods on its Class object will return an array of Method objects that include all of the methods in those interfaces, and invoking getMethod will find methods in the proxy interfaces as would be expected.
• The Proxy.isProxyClass method will return true if it is passed a proxy class— a class returned by Proxy.getProxyClass or the class of an object returned by Proxy.newProxyInstance — and false otherwise.
• The java.security.ProtectionDomain of a proxy class is the same as that of system classes loaded by the bootstrap class loader, such as java.lang.Object , because the code for a proxy class is generated by trusted system code. This protection domain will typically be granted java.security.AllPermission .
• Each proxy class has one public constructor that takes one argument, an implementation of the interface InvocationHandler , to set the invocation handler for a proxy instance. Rather than having to use the reflection API to access the public constructor, a proxy instance can be also be created by calling the Proxy.newProxyInstance method, which combines the actions of calling Proxy.getProxyClass with invoking the constructor with an invocation handler.

Methods Duplicated in Multiple Proxy Interfaces

When two or more interfaces of a proxy class contain a method with the same name and parameter signature, the order of the proxy class’s interfaces becomes significant. When such a duplicate method is invoked on a proxy instance, the Method object passed to the invocation handler will not necessarily be the one whose declaring class is assignable from the reference type of the interface that the proxy’s method was invoked through. This limitation exists because the corresponding method implementation in the generated proxy class cannot determine which interface it was invoked through. Therefore, when a duplicate method is invoked on a proxy instance, the Method object for the method in the foremost interface that contains the method (either directly or inherited through a superinterface) in the proxy class’s list of interfaces is passed to the invocation handler’s invoke method, regardless of the reference type through which the method invocation occurred. If a proxy interface contains a method with the same name and parameter signature as the hashCode , equals , or toString methods of java.lang.Object , when such a method is invoked on a proxy instance, the Method object passed to the invocation handler will have java.lang.Object as its declaring class. In other words, the public, non-final methods of java.lang.Object logically precede all of the proxy interfaces for the determination of which Method object to pass to the invocation handler. Note also that when a duplicate method is dispatched to an invocation handler, the invoke method may only throw checked exception types that are assignable to one of the exception types in the throws clause of the method in all of the proxy interfaces that it can be invoked through. If the invoke method throws a checked exception that is not assignable to any of the exception types declared by the method in one of the proxy interfaces that it can be invoked through, then an unchecked UndeclaredThrowableException will be thrown by the invocation on the proxy instance. This restriction means that not all of the exception types returned by invoking getExceptionTypes on the Method object passed to the invoke method can necessarily be thrown successfully by the invoke method.

Field Summary

Constructor Summary

Constructs a new Proxy instance from a subclass (typically, a dynamic proxy class) with the specified value for its invocation handler.

Method Summary

Returns the java.lang.Class object for a proxy class given a class loader and an array of interfaces.

Returns true if and only if the specified class was dynamically generated to be a proxy class using the getProxyClass method or the newProxyInstance method.

Returns an instance of a proxy class for the specified interfaces that dispatches method invocations to the specified invocation handler.

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