Java ClassLoader 基本使用笔记

TestClassLoader.java

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


/**
 * 项目中需要加载第三方提供的Java类，就先熟悉一下ClassLoader。
 * 下面的测试：
 *   1. 从指定的jar或class目录中加载相应的class。
 *   2. 新定义的classLoader能继承现有的classLoader
 *
 * TODO:
 *   1. 如何 unload/reload 已经加载的class？貌似OSGI作的更好吧。
 *   每个class内部是保持了对加载它的ClassLoader的引用的。
 *   每个对象实例与其对应的class之间又是何种关系？只能通过每次新建一个classLoader，让旧的被gc么？
 *
 *   2. 如何应用 SecurityManager？
 *
 * 参考：
 *
 *
 *
 */
public class TestClassLoader {

    public static void main(String[] args) throws Exception {
        System.out.println("---------------------------");
        testJar();

        System.out.println("---------------------------");
        testClassFolader();
    }

    @SuppressWarnings("unchecked")
    public static void testJar() throws Exception {
        String s = "F:/java/Apache Commons/commons-lang-2.3.jar";
        File f = new File(s);
        URL url = f.toURI().toURL();
        System.out.println(url);
        URLClassLoader classLoader = new URLClassLoader(new URL[] { url },
                TestClassLoader.class.getClassLoader());
        Class c = classLoader.loadClass("org.apache.commons.lang.StringUtils");
        System.out.println(c);
        c = classLoader.loadClass("me.test.AAA"); // in current project "A"
        System.out.println(c);
        printClassPath(classLoader);
    }

    @SuppressWarnings("unchecked")
    public static void testClassFolader() throws Exception {
        String s = "E:/workspace/A1/build/classes";
        File f = new File(s);
        URL url = f.toURI().toURL();
        System.out.println(url);
        URLClassLoader classLoader = new URLClassLoader(new URL[] { url },
                TestClassLoader.class.getClassLoader());
        Class c = classLoader.loadClass("me.test.BBB"); // in another project "A1"
        System.out.println(c);
        c = classLoader.loadClass("me.test.AAA");
        System.out.println(c);
        printClassPath(classLoader);
    }

    public static void printClassPath(URLClassLoader clsLoader)
            throws Exception {
        URLClassLoader loader = clsLoader;
        if (clsLoader == null) {
            loader = (URLClassLoader) ClassLoader.getSystemClassLoader();
        }
        URL[] urls = loader.getURLs();
        for (int i = 0; i < urls.length; i++) {
            System.out.println(urls[i].getFile());
        }
        if (clsLoader.getParent() != null) {
            System.out.println("-----");
            printClassPath((URLClassLoader) clsLoader.getParent());
        }
    }

}

运行结果：

---------------------------
file:/F:/java/Apache%20Commons/commons-lang-2.3.jar
class org.apache.commons.lang.StringUtils
class me.test.AAA
/F:/java/Apache%20Commons/commons-lang-2.3.jar
-----
/E:/stock-workspace/A/bin/
-----
/E:/DevIDE/jdk1.6.0_20/jre/lib/ext/dnsns.jar
/E:/DevIDE/jdk1.6.0_20/jre/lib/ext/localedata.jar
/E:/DevIDE/jdk1.6.0_20/jre/lib/ext/sunjce_provider.jar
/E:/DevIDE/jdk1.6.0_20/jre/lib/ext/sunmscapi.jar
/E:/DevIDE/jdk1.6.0_20/jre/lib/ext/sunpkcs11.jar
---------------------------
file:/E:/workspace/A1/build/classes/
class me.test.BBB
class me.test.AAA
/E:/workspace/A1/build/classes/
-----
/E:/stock-workspace/A/bin/
-----
/E:/DevIDE/jdk1.6.0_20/jre/lib/ext/dnsns.jar
/E:/DevIDE/jdk1.6.0_20/jre/lib/ext/localedata.jar
/E:/DevIDE/jdk1.6.0_20/jre/lib/ext/sunjce_provider.jar
/E:/DevIDE/jdk1.6.0_20/jre/lib/ext/sunmscapi.jar
/E:/DevIDE/jdk1.6.0_20/jre/lib/ext/sunpkcs11.jar