自定義ClassLoader的簡單例子

Test1.java:

package test;

import java.io.*;
import java.lang.reflect.*;

class AClassLoader extends ClassLoader {
    public Class loadClass(String name, boolean resolve) throws ClassNotFoundException {
        Class klass = null;
        try {
            klass = findLoadedClass(name); //檢查該類是否已經被裝載。
            if (klass != null) {
                return klass;  
            }
           
            byte[] bs = getClassBytes(name);//從一個特定的信息源尋找並讀取該類的字節。
            if (bs != null && bs.length > 0) {
                klass = defineClass(name, bs, 0, bs.length);  
            }
            if (klass == null) { //如果讀取字節失敗，則試圖從JDK的系統API中尋找該類。
                klass = findSystemClass(name);
            }
            if (resolve && klass != null) {
                resolveClass(klass);  
            }
        } catch (IOException e) {
            throw new ClassNotFoundException(e.toString());
        }  
        System.out.println("klass == " + klass);
        return klass;
    }  
    private byte[] getClassBytes(String className) throws IOException {
        String path = System.getProperty("java.class.path") + File.separator;
        path += className.replace('.', File.separatorChar) + ".class";
        System.out.println(path);
        FileInputStream fis = null;
        try {
            fis = new FileInputStream(path);
        } catch (FileNotFoundException e) {
            System.out.println(e);
            return null;   //如果查找失敗，則放棄查找。捕捉這個異常主要是爲了過濾JDK的系統API。
        }
        byte[] bs = new byte[fis.available()];
        fis.read(bs);
        return bs;
    }
}

public class Test1 {
    public static void main(String[] args) throws Exception{
        AClassLoader loader = new AClassLoader();
        Class c = loader.loadClass("test.A", false);
        System.out.println(c);
        Object o = c.newInstance();
        System.out.println(o);
        Method m = c.getMethod("print", java.lang.String.class);
        m.invoke(o, "bitan");
    }  
}

 

 

A.java

package test;
public class A {
    public void print(String str) {
        System.out.println("嗨，" + str + ", 你終於找到我了！");  
    }
    public String toString() {
        return "我是一個A對象！";  
    }  
}

 

結果：

 

C:/test>javac -d ./bin ./src/*.java

C:/test>java -classpath ./bin test.Test1
./bin/test/A.class
./bin/java/lang/Object.class
java.io.FileNotFoundException: ./bin/java/lang/Object.class (系統找不到指定的路
徑。)
klass == class java.lang.Object
klass == class test.A
class test.A
我是一個A對象！
./bin/java/lang/String.class
java.io.FileNotFoundException: ./bin/java/lang/String.class (系統找不到指定的路
徑。)
klass == class java.lang.String
./bin/java/lang/System.class
java.io.FileNotFoundException: ./bin/java/lang/System.class (系統找不到指定的路
徑。)
klass == class java.lang.System
./bin/java/lang/StringBuilder.class
java.io.FileNotFoundException: ./bin/java/lang/StringBuilder.class (系統找不到指
定的路徑。)
klass == class java.lang.StringBuilder
./bin/java/io/PrintStream.class
java.io.FileNotFoundException: ./bin/java/io/PrintStream.class (系統找不到指定的
路徑。)
klass == class java.io.PrintStream
嗨，bitan, 你終於找到我了！

C:/test>