Java動態代理的應用詳解

動態代理其實就是java.lang.reflect.Proxy類別動態的根據您指定的所有介面產生一個class byte，該class會繼承Proxy類，並實作所有你指定的介面（您在參數中傳入的介面陣列）；然後再利用您指定的classloader將 class byte載入進系統，最後產生這樣一個類別的對象，並初始化該對象的一些值，如invocationHandler,以即所有的介面對應的Method成員。 初始化之後將物件傳回給呼叫的客戶端。這樣客戶端拿到的就是一個實作你所有的介面的Proxy物件。請看實例分析：

package com.fans.common.proxy;
public interface BusinessProcessor {
  public void processBusiness();
}

package com.fans.common.proxy;
/**
 * 业务处理类
 * @author fanshadoop
 *
 */
public class BusinessProcessorImpl implements BusinessProcessor {
 @Override
 public void processBusiness() {
  System.out.println("processing business.....");
 }
}

package com.fans.common.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
/**
 * 业务代理类
 * @author fanshadoop
 *
 */
public class BusinessProcessorHandler implements InvocationHandler {
 private Object target = null;
 BusinessProcessorHandler(Object target) {
  this.target = target;
 }
 public Object invoke(Object proxy, Method method, Object[] args)
   throws Throwable {
  System.out
    .println("You can do something here before process your business");
  Object result = method.invoke(target, args);
  System.out
    .println("You can do something here after process your business");
  return result;
 }
}

package com.fans.common.proxy;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.Proxy;
public class Test {
 /**
  * @param args
  */
 public static void main(String[] args) {
  BusinessProcessorImpl bpimpl = new BusinessProcessorImpl();
  BusinessProcessorHandler handler = new BusinessProcessorHandler(bpimpl);
  BusinessProcessor bp = (BusinessProcessor) Proxy.newProxyInstance(
    bpimpl.getClass().getClassLoader(), bpimpl.getClass()
      .getInterfaces(), handler);
  bp.processBusiness();
  System.out.println(bp.getClass().getName());
  printClassDefinition(bp.getClass());
 }
 public static String getModifier(int modifier) {
  String result = "";
  switch (modifier) {
  case Modifier.PRIVATE:
   result = "private";
  case Modifier.PUBLIC:
   result = "public";
  case Modifier.PROTECTED:
   result = "protected";
  case Modifier.ABSTRACT:
   result = "abstract";
  case Modifier.FINAL:
   result = "final";
  case Modifier.NATIVE:
   result = "native";
  case Modifier.STATIC:
   result = "static";
  case Modifier.SYNCHRONIZED:
   result = "synchronized";
  case Modifier.STRICT:
   result = "strict";
  case Modifier.TRANSIENT:
   result = "transient";
  case Modifier.VOLATILE:
   result = "volatile";
  case Modifier.INTERFACE:
   result = "interface";
  }
  return result;
 }
 public static void printClassDefinition(Class clz) {
  String clzModifier = getModifier(clz.getModifiers());
  if (clzModifier != null && !clzModifier.equals("")) {
   clzModifier = clzModifier + " ";
  }
  String superClz = clz.getSuperclass().getName();
  if (superClz != null && !superClz.equals("")) {
   superClz = "extends " + superClz;
  }
  Class[] interfaces = clz.getInterfaces();
  String inters = "";
  for (int i = 0; i < interfaces.length; i++) {
   if (i == 0) {
    inters += "implements ";
   }
   inters += interfaces[i].getName();
  }
  System.out.println(clzModifier + clz.getName() + " " + superClz + " "
    + inters);
  System.out.println("{");
  Field[] fields = clz.getDeclaredFields();
  for (int i = 0; i < fields.length; i++) {
   String modifier = getModifier(fields[i].getModifiers());
   if (modifier != null && !modifier.equals("")) {
    modifier = modifier + " ";
   }
   String fieldName = fields[i].getName();
   String fieldType = fields[i].getType().getName();
   System.out.println("    " + modifier + fieldType + " " + fieldName
     + ";");
  }
  System.out.println();
  Method[] methods = clz.getDeclaredMethods();
  for (int i = 0; i < methods.length; i++) {
   Method method = methods[i];
   String modifier = getModifier(method.getModifiers());
   if (modifier != null && !modifier.equals("")) {
    modifier = modifier + " ";
   }
   String methodName = method.getName();
   Class returnClz = method.getReturnType();
   String retrunType = returnClz.getName();
   Class[] clzs = method.getParameterTypes();
   String paraList = "(";
   for (int j = 0; j < clzs.length; j++) {
    paraList += clzs[j].getName();
    if (j != clzs.length - 1) {
     paraList += ", ";
    }
   }
   paraList += ")";
   clzs = method.getExceptionTypes();
   String exceptions = "";
   for (int j = 0; j < clzs.length; j++) {
    if (j == 0) {
     exceptions += "throws ";
    }
    exceptions += clzs[j].getName();
    if (j != clzs.length - 1) {
     exceptions += ", ";
    }
   }
   exceptions += ";";
   String methodPrototype = modifier + retrunType + " " + methodName
     + paraList + exceptions;
   System.out.println("    " + methodPrototype);
  }
  System.out.println("}");
 }
}

運行結果：

You can do something here before process your business
processing business.....
You can do something here after process your business
$Proxy0
$Proxy0 extends java.lang.reflect.Proxy implements com.fans.common.proxy.BusinessProcessor
{
    java.lang.reflect.Method m1;
    java.lang.reflect.Method m3;
    java.lang.reflect.Method m0;
    java.lang.reflect.Method m2;
    boolean equals(java.lang.Object);
    java.lang.String toString();
    int hashCode();
    void processBusiness();
}

類BusinessProcessorHandler實作了InvocationHandler介面的invoke方法，這個類別就是Proxy最終呼叫固定介面方法。

很明顯，Proxy.newProxyInstance方法會做以下幾件事：
    1，根據傳入的第二個參數interfaces動態產生一個類，實作interfaces中的接口，該例中即BusinessProcessor接口的processBusiness方法。並且繼承了Proxy類，重寫了hashcode,toString,equals等三個方法。具體實現可參看 ProxyGenerator.generateProxyClass(...); 此範例中產生了$Proxy0類別
    2，並透過傳入的第一個參數classloder將剛產生的類別載入jvm。即將$Proxy0類別load
    3，利用第三個參數，呼叫$Proxy0的$Proxy0(InvocationHandler)建構子建立$Proxy0的對象，並且用interfaces參數遍歷其所有介面的方法，並產生Method對像初始化物件的幾個Method成員變數
    4，將$Proxy0的執行個體傳回給客戶端。
現在好了。我們再看客戶端怎麼調就清楚了。
    1，客戶端拿到的是$Proxy0的實例對象，由於$Proxy0繼承了BusinessProcessor，因此轉換為BusinessProcessor沒任何問題。
         BusinessProcessor bp = (BusinessProcessor)Proxy.newProxyInstance(....);
    2，bp.processBusiness()； ()的實現就是透過InvocationHandler去呼叫invoke方法啦!

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