ClassLoader 与 URLClassLoader 的用法！

转自： http://blog.csdn.net/suiusoar/article/details/7530191

1. 读取配置文件中的信息

配置文件的内容：

用URLClassLoader读配置文件内容：

 import java.io.IOException;
 import java.net.URL;
 import java.net.URLClassLoader;
 import java.util.Properties;

 public class Testext {
 public static void main(String[] args) throws IOException {
Properties properties = new Properties();
URL[] urls = new URL[] { new URL("file:/"
+ System.getProperty("user.dir") + "/ravageWeb/") };
URLClassLoader ucl = new URLClassLoader(urls);
//ClassLoader cl = Testext.class.getClassLoader(); //这两行也可以不写
//cl = ucl; //如果写，下面就要改为：cl.getResourceAsStream...
properties.load(ucl.getResourceAsStream("resources/testpro.properties"));
System.out.println(properties.getProperty("sui"));
}
}

import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.Properties;

public class Testext {
	public static void main(String[] args) throws IOException {
		Properties properties = new Properties();
		URL[] urls = new URL[] { new URL("file:/"
				+ System.getProperty("user.dir") + "/ravageWeb/") };
		URLClassLoader ucl = new URLClassLoader(urls);
		//ClassLoader cl = Testext.class.getClassLoader(); 	//这两行也可以不写
		//cl = ucl; 	//如果写，下面就要改为：cl.getResourceAsStream...
		properties.load(ucl.getResourceAsStream("resources/testpro.properties"));
		System.out.println(properties.getProperty("sui"));
	}
}

遇到的问题：1. "/ravageWeb/" 中第二个 / 可写可不写。 但是若改为 "/ravageWeb/resources/" 则resources后必须加 / 否则报NullPointerException

用ClassLoader读配置文件内容：

import java.io.IOException;
import java.util.Properties;

public class Testext {
public static void main(String[] args) throws IOException {
Properties properties = new Properties();
properties.load(Testext.class.getClassLoader()
.getResourceAsStream("resources/testpro.properties"));
System.out.println(properties.getProperty("sui"));
}
}

import java.io.IOException;
import java.util.Properties;

public class Testext {
	public static void main(String[] args) throws IOException {
		Properties properties = new Properties();
		properties.load(Testext.class.getClassLoader()
				.getResourceAsStream("resources/testpro.properties"));
		System.out.println(properties.getProperty("sui"));
	}
}

 

总结：很显然ClassLoader与URLClassLoader的区别是：

ClassLoader只能读位于classpath下的文件（即：resources/testpro.properties必须放到src目录下，编译后进入bin目录）；

而URLClassLoader可以读任意目录下的文件！

2. 载入一个类

用ClassLoader载入一个类：

import java.io.IOException;

public class Testext {
public static void main(String[] args) throws IOException,
ClassNotFoundException {
Class c = Testext.class.getClassLoader().loadClass("stalk.STalkClient");
System.out.println(c.getName()); //输出：stalk.STalkClient
}
}

import java.io.IOException;

public class Testext {
	public static void main(String[] args) throws IOException,
			ClassNotFoundException {
		Class c = Testext.class.getClassLoader().loadClass("stalk.STalkClient");
		System.out.println(c.getName());	//输出：stalk.STalkClient
	}
}

 

用URLClassLoader载入一个类：

import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;

public class Testext {
public static void main(String[] args) throws IOException,
ClassNotFoundException {

URL[] urls = new URL[] { new URL("file:/"
+ System.getProperty("user.dir") + "/ravageWeb/") };
URLClassLoader ucl = new URLClassLoader(urls);
Class c = ucl.loadClass("NoPackage");
System.out.println(c.getName());
}
}

import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;

public class Testext {
	public static void main(String[] args) throws IOException,
			ClassNotFoundException {
		
		URL[] urls = new URL[] { new URL("file:/"
				+ System.getProperty("user.dir") + "/ravageWeb/") };
		URLClassLoader ucl = new URLClassLoader(urls);
		Class c = ucl.loadClass("NoPackage");
		System.out.println(c.getName());
	}
}

遇到的问题："/ravageWeb/" 中第二个 / 必须写，否则报ClassNotFoundException

 

总结：

ClassLoader只能load位于classpath（src目录）下的类；

而URLClassLoader可以load任意目录下的类！