java动态加载jar包，并运行其中的类和方法 （jar是与平台无关的文件格式）

ecplise直接另存为就可以保存为jar格式

JAR（Java Archive，Java 归档文件）是与平台无关的文件格式，它允许将许多文件组合成一个压缩文件

动态加载jar包，在实际开发中经常会需要用到，尤其涉及平台和业务的关系的时候，业务逻辑部分可以独立出去交给业务方管理，业务方只需要提供jar包，就能在平台上运行。

下面通过一个实例来直观演示：

第一：定义一个抽象类 AbstractAction (稍后换成接口的实例)

package com.java.loader;

public abstract class AbstractAction {
    public abstract String action();
}

package com.java.loader;

public abstract class AbstractAction {
	public abstract String action();
}

第二：写个实体类继承一下 TestAction

package com.java.jarloader;

import com.java.loader.AbstractAction;

public class TestAction extends AbstractAction{
    public String action() {
        System.out.println("I am working ! ");
        return "this ActionTest class";
    }
}

package com.java.jarloader;

import com.java.loader.AbstractAction;

public class TestAction extends AbstractAction{
	public String action() {
		System.out.println("I am working ! ");
		return "this ActionTest class";
	}
}

第三：将TestAction所在的包导出成jar包的方式，eclipse中直接export即可，放到指定目录，此处放在

D:\jarload\test.jar

D:\jarload\test.jar

放好后，删除TestAction文件和package。

第四：写个main函数测试下 TestMain (这里比较无聊，写了个从文件读的方式获取jar路劲，路径就是上面提到的jar所在的位置)

package com.java.main;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.net.URL;
import java.net.URLClassLoader;

import com.java.loader.AbstractAction;
import com.java.loader.AbstractAction;


/**
 *
 * 两种方式
 * @author minggang.wumg
 *
 */

public class TestMain {

    public static void main(String[] args) {
        try {
            //第一种  配置成文件格式
            File file = new File("D:\\jarload\\test.txt");
            BufferedReader in = new BufferedReader(new FileReader(file));
            String s = new String();
            while ((s = in.readLine()) != null) {

                URL url = new URL(s);
                s = null;

                URLClassLoader myClassLoader = new URLClassLoader(new URL[] { url }, Thread.currentThread()
                        .getContextClassLoader());
                Class<? extends AbstractAction> myClass = (Class<? extends AbstractAction>) myClassLoader.loadClass("com.java.jarloader.TestAction");
                AbstractAction action = (AbstractAction) myClass.newInstance();
                String str = action.action();
                System.out.println(str);

                //第二种
                URL url1 = new URL("file:D:/jarload/test.jar");
                URLClassLoader myClassLoader1 = new URLClassLoader(new URL[] { url1 }, Thread.currentThread()
                        .getContextClassLoader());
                Class<?> myClass1 = myClassLoader1.loadClass("com.java.jarloader.TestAction");
                AbstractAction action1 = (AbstractAction) myClass1.newInstance();
                String str1 = action1.action();
                System.out.println(str1);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

package com.java.main;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.net.URL;
import java.net.URLClassLoader;

import com.java.loader.AbstractAction;
import com.java.loader.AbstractAction;


/**
 * 
 * 两种方式
 * @author minggang.wumg
 *
 */

public class TestMain {

	public static void main(String[] args) {
		try {
			//第一种  配置成文件格式
			File file = new File("D:\\jarload\\test.txt");
			BufferedReader in = new BufferedReader(new FileReader(file));
			String s = new String();
			while ((s = in.readLine()) != null) {

				URL url = new URL(s);
				s = null;
				
				URLClassLoader myClassLoader = new URLClassLoader(new URL[] { url }, Thread.currentThread()
						.getContextClassLoader());
				Class<? extends AbstractAction> myClass = (Class<? extends AbstractAction>) myClassLoader.loadClass("com.java.jarloader.TestAction");
				AbstractAction action = (AbstractAction) myClass.newInstance();
				String str = action.action();
				System.out.println(str);
				
				//第二种
				URL url1 = new URL("file:D:/jarload/test.jar");
				URLClassLoader myClassLoader1 = new URLClassLoader(new URL[] { url1 }, Thread.currentThread()
						.getContextClassLoader());
				Class<?> myClass1 = myClassLoader1.loadClass("com.java.jarloader.TestAction");
				AbstractAction action1 = (AbstractAction) myClass1.newInstance();
				String str1 = action1.action();
				System.out.println(str1);
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

}

第五：运行结果：

这是能完成运行的！

下面我们来改写下：

第一：将抽象类改成接口的形式  InterfaceAction 

package com.java.loader;

public interface InterfaceAction {
    public String action();
}

package com.java.loader;

public interface InterfaceAction {
	public String action();
}

第二：改写下实体类，实现接口 TestAction

package com.java.jarloader;

import com.java.loader.InterfaceAction;

public class TestAction implements InterfaceAction{

    @Override
    public String action() {
        System.out.println("I am working ! ");
        return "this ActionTest class";
    }

}

package com.java.jarloader;

import com.java.loader.InterfaceAction;

public class TestAction implements InterfaceAction{

	@Override
	public String action() {
		System.out.println("I am working ! ");
		return "this ActionTest class";
	}

}

第三步相同。

第四步：稍作修改 TestMain

package com.java.main;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.net.URL;
import java.net.URLClassLoader;

import javax.swing.AbstractAction;

import com.java.loader.InterfaceAction;




/**
 *
 * 两种方式
 * @author minggang.wumg
 *
 */

public class TestMain {

    public static void main(String[] args) {
        try {
            //第一种  配置成文件格式
            File file = new File("D:\\jarload\\test.txt");
            BufferedReader in = new BufferedReader(new FileReader(file));
            String s = new String();
            while ((s = in.readLine()) != null) {

                URL url = new URL(s);
                s = null;

                URLClassLoader myClassLoader = new URLClassLoader(new URL[] { url }, Thread.currentThread()
                        .getContextClassLoader());
                Class<?> myClass = (Class<?>) myClassLoader.loadClass("com.java.jarloader.TestAction");
                InterfaceAction action = (InterfaceAction) myClass.newInstance();
                String str = action.action();
                System.out.println(str);

                //第二种
                URL url1 = new URL("file:D:/jarload/test.jar");
                URLClassLoader myClassLoader1 = new URLClassLoader(new URL[] { url1 }, Thread.currentThread()
                        .getContextClassLoader());
                Class<?> myClass1 = myClassLoader1.loadClass("com.java.jarloader.TestAction");
                InterfaceAction action1 = (InterfaceAction) myClass1.newInstance();
                String str1 = action1.action();
                System.out.println(str1);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

package com.java.main;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.net.URL;
import java.net.URLClassLoader;

import javax.swing.AbstractAction;

import com.java.loader.InterfaceAction;




/**
 * 
 * 两种方式
 * @author minggang.wumg
 *
 */

public class TestMain {

	public static void main(String[] args) {
		try {
			//第一种  配置成文件格式
			File file = new File("D:\\jarload\\test.txt");
			BufferedReader in = new BufferedReader(new FileReader(file));
			String s = new String();
			while ((s = in.readLine()) != null) {

				URL url = new URL(s);
				s = null;
				
				URLClassLoader myClassLoader = new URLClassLoader(new URL[] { url }, Thread.currentThread()
						.getContextClassLoader());
				Class<?> myClass = (Class<?>) myClassLoader.loadClass("com.java.jarloader.TestAction");
				InterfaceAction action = (InterfaceAction) myClass.newInstance();
				String str = action.action();
				System.out.println(str);
				
				//第二种
				URL url1 = new URL("file:D:/jarload/test.jar");
				URLClassLoader myClassLoader1 = new URLClassLoader(new URL[] { url1 }, Thread.currentThread()
						.getContextClassLoader());
				Class<?> myClass1 = myClassLoader1.loadClass("com.java.jarloader.TestAction");
				InterfaceAction action1 = (InterfaceAction) myClass1.newInstance();
				String str1 = action1.action();
				System.out.println(str1);
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

}

第五：运行结果相同。