Posted on 2010-08-16 10:55
java小爬蟲 閱讀(1633)
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如上文：代理模式和裝飾者模式中的靜態代理實例，它具有如下缺陷：
1：代理類不可重用，具有相同代理邏輯的類會大量產生；
2：被代理方法惟一，如果有多個方法都需要相同邏輯的代理，那么代理類中就有大量的相似的方法存在；
3：代理方法不具有參數；
4：只實現了單接口了代理；
所以它并不具有實戰意義上的價值！
那么，這些問題該如何解決呢？動態代理又是一步一步如何演變過來的呢？
下面就讓我們來一步一步以實例的方式來探究它的演變的細節。
如果我們能動態產生一個代理類的源文件，編譯后加載到內存，那么我們就可以獲取到動態的代理對象。
package proxy;
import java.io.File;
import java.io.FileWriter;
import java.lang.reflect.Constructor;
import java.net.URL;
import java.net.URLClassLoader;
import javax.tools.JavaCompiler;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;
import javax.tools.JavaCompiler.CompilationTask;
public class Proxy {
public static Object newProxyInstance(Object target) throws Exception { //JDK6 Complier API, CGLib, ASM
String rt = "\r\n";
String t = "\t";
String src ="package proxy;"+ rt + t +
"public class TankTimeProxy implements Movable {"+ rt + t +
"private Movable obj; "+ rt + t +
"public TankTimeProxy(Movable obj) {"+ rt + t +
"super();"+ rt + t +
"this.obj = obj;"+ rt + t +
"}"+ rt + t +
"@Override"+ rt + t +
"public void move() {"+ rt + t +
"long begintime = System.currentTimeMillis();"+ rt + t +
"System.out.println(\" Tank is begining to move !\");"+ rt + t +
"obj.move();"+ rt + t +
"long endtime = System.currentTimeMillis();"+ rt + t +
"System.out.println(\" Tank is stop !\");"+ rt + t +
"System.out.println(\"move time : \"+(endtime-begintime));"+ rt + t +
"}"+ rt + t +
"}";
String fileName =System.getProperty("user.dir")+"/src/proxy/TankTimeProxy.java";
File f = new File(fileName);
FileWriter fw = new FileWriter(f);
if(f.exists()){
f.delete();
fw.flush();
f = new File(fileName);
};
fw.write(src);
fw.flush();
fw.close();
//compile
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fileMgr = compiler.getStandardFileManager(null, null, null);
Iterable units = fileMgr.getJavaFileObjects(fileName);
CompilationTask compilationTask = compiler.getTask(null, fileMgr, null, null, null, units);
compilationTask.call();
fileMgr.close();
//load into memory and create an instance
URL[] urls = new URL[] {new URL("file:/" + System.getProperty("user.dir")+ "/src/")};
URLClassLoader ul = new URLClassLoader(urls);
Class c = ul.loadClass("proxy.TankTimeProxy");
Constructor ctr = c.getConstructor(target.getClass().getInterfaces()[0]);
Object m = ctr.newInstance(target);
return m;
}
}
測試類：
package proxy;
public class Client {
public static void main(String[] args) throws Exception{
Movable tank = (Movable)Proxy.newProxyInstance(new Tank());
tank.move();
}
}
上面的類就實現了生成
TankTimeProxy.java文件，編譯，加載并被調用的功能。（全部源碼見：代理模式和裝飾者模式異同點比較
）
那么怎么實現代理任意對象呢？
在產生代理類的時候，只要動態的注入目標對象，就實現了對任意對象的代理。
怎么實現對任意方法的代理呢？通過java反射機制，可以獲取一個類的所有方法，即可以獲取目標類的所有方法，在組成代理類java源碼的時候，循環遍歷嵌入處理邏輯就可以任意對多方法的代理了。
示例代碼如下：
package proxy;
import java.io.File;
import java.io.FileWriter;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import javax.tools.JavaCompiler;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;
import javax.tools.JavaCompiler.CompilationTask;
public class Proxy 
{
public static Object newProxyInstance(Class intface,Object target) throws Exception { //JDK6 Complier API, CGLib, ASM
String rt = "\r\n";
String t = "\t";
String methodStr = "" ;
Method[] methods = intface.getDeclaredMethods();
for (Method method : methods) {
methodStr += "@Override"+ rt + t +
"public void "+method.getName()+"() {"+ rt + t +
"long begintime = System.currentTimeMillis();"+ rt + t +
"System.out.println(\" Tank is begining to move !\");"+ rt + t +
"obj."+method.getName()+"();"+ rt + t +
"long endtime = System.currentTimeMillis();"+ rt + t +
"System.out.println(\" Tank is stop !\");"+ rt + t +
"System.out.println(\"move time : \"+(endtime-begintime));"+ rt + t +
"}"+ rt + t;
}
System.out.println(methodStr);
String src ="package proxy;"+ rt + t +
"import "+intface.getName()+";"+ rt + t +
"public class $Proxy1 implements "+intface.getSimpleName()+" {"+ rt + t +
"private "+intface.getSimpleName()+" obj; "+ rt + t +
"public $Proxy1("+intface.getSimpleName()+" obj) {"+ rt + t +
"super();"+ rt + t +
"this.obj = obj;"+ rt + t +
"}" + rt + t +
methodStr +rt +
"}";
String fileName =System.getProperty("user.dir")+"/src/proxy/$Proxy1.java";
File f = new File(fileName);
FileWriter fw = new FileWriter(f);
if(f.exists()){
f.delete();
fw.flush();
f = new File(fileName);
};
fw.write(src);
fw.flush();
fw.close();
//compile
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fileMgr = compiler.getStandardFileManager(null, null, null);
Iterable units = fileMgr.getJavaFileObjects(fileName);
CompilationTask compilationTask = compiler.getTask(null, fileMgr, null, null, null, units);
compilationTask.call();
fileMgr.close();
//load into memory and create an instance
URL[] urls = new URL[] {new URL("file:/" + System.getProperty("user.dir")+ "/src/")};
URLClassLoader ul = new URLClassLoader(urls);
Class c = ul.loadClass("proxy.$Proxy1");
Constructor ctr = c.getConstructor(intface);
Object m = ctr.newInstance(target);
return m;
}
}
但是新的問題又出現了：
在代理類中，被代理方法前后的處理邏輯已經被“寫死了”，很難改變增加的功能，這又該如何處理呢？
我們可以這樣考慮，增加一個調用處理器InvocationHandler，把對方法的處理邏輯進行進一步的封閉，并把InvocationHandler分離出來，如果可以的話，就實現了對代理邏輯的可修改性。
那么InvocationHandler里面應該封閉些什么東西呢？
for (Method method : methods) {
methodStr += "@Override"+ rt + t +
"public void "+method.getName()+"() {"+ rt + t +
"long begintime = System.currentTimeMillis();"+ rt + t +
"System.out.println(\" Tank is begining to move !\");"+ rt + t +
"obj."+method.getName()+"();"+ rt + t +
"long endtime = System.currentTimeMillis();"+ rt + t +
"System.out.println(\" Tank is stop !\");"+ rt + t +
"System.out.println(\"move time : \"+(endtime-begintime));"+ rt + t +
"}"+ rt + t;
}
從這個片段代理可以看出，我們應該在InvocationHandler中封裝obj對象，即被代理類的接口（實現類）。還應該實現：被分離出去的InvocationHandler能被代理類調用，我們應該把InvocationHandler聚合進來。
代碼演變示例：
package proxy;
import java.lang.reflect.Method;
public interface InvocationHandler {
public void invoke(Object proxy, Method m);
}
package proxy;
import java.lang.reflect.Method;
public class TimeHandler implements InvocationHandler{
private Object target;
public TimeHandler(Object target) {
this.target = target;
}
@Override
public void invoke(Object o, Method m) {
long start = System.currentTimeMillis();
System.out.println("starttime:" + start);
System.out.println(o.getClass().getName());
try {
m.invoke(target);
} catch (Exception e) {
e.printStackTrace();
}
long end = System.currentTimeMillis();
System.out.println("time:" + (end-start));
System.out.println("endtime:" + end);
}
}
package proxy;
import java.io.File;
import java.io.FileWriter;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;
import javax.tools.JavaCompiler;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;
import javax.tools.JavaCompiler.CompilationTask;
public class Proxy {
public static Object newProxyInstance(Class intface,InvocationHandler h) throws Exception { //JDK6 Complier API, CGLib, ASM
String rt = "\r\n";
String t = "\t";
String methodStr = "" ;
Method[] methods = intface.getDeclaredMethods();
for(Method method : methods) {
methodStr += "@Override" + rt + t + t +
"public void " + method.getName() + "() {" + rt + t + t + t +
"try {" + rt + t + t + t + t +
"Method md = " + intface.getName() + ".class.getMethod(\"" + method.getName() + "\");" + rt + t + t + t + t +
"h.invoke(this, md);" + rt + t + t + t +
"}catch(Exception e) {" + rt + t + t + t +
"e.printStackTrace();" + rt + t + t +
"}" + rt + t +
"}" + rt + t + t ;
}
System.out.println(methodStr);
String src ="package proxy;"+ rt + t +
"import "+intface.getName()+";"+ rt + t +
"import java.lang.reflect.Method;"+ rt + t +
"public class $Proxy1 implements "+intface.getSimpleName()+" {"+ rt + t +
"private InvocationHandler h ; "+ rt + t +
"public $Proxy1(InvocationHandler h) {"+ rt + t +
"super();"+ rt + t +
"this.h = h;"+ rt + t +
"}" + rt + t +
methodStr +rt +
"}";
String fileName =System.getProperty("user.dir")+"/src/proxy/$Proxy1.java";
File f = new File(fileName);
FileWriter fw = new FileWriter(f);
if(f.exists()){
f.delete();
fw.flush();
f = new File(fileName);
};
fw.write(src);
fw.flush();
fw.close();
//compile
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fileMgr = compiler.getStandardFileManager(null, null, null);
Iterable units = fileMgr.getJavaFileObjects(fileName);
CompilationTask compilationTask = compiler.getTask(null, fileMgr, null, null, null, units);
compilationTask.call();
fileMgr.close();
//load into memory and create an instance
URL[] urls = new URL[] {new URL("file:/" + System.getProperty("user.dir")+ "/src/")};
URLClassLoader ul = new URLClassLoader(urls);
Class c = ul.loadClass("proxy.$Proxy1");
Constructor ctr = c.getConstructor(InvocationHandler.class);
Object m = ctr.newInstance(h);
return m;
}
}
這樣，就實現了對任意對象，任意方法的代理。
缺點就是：多接口代理沒有實現，被代理對象的方法沒有支持參數。