Java ClassLoader深入研究

参考文章：
http://blog.csdn.net/lovingprince/archive/2009/06/03/4238695.aspx

http://www.yesky.com/243/1840743.shtml
http://blog.csdn.net/gtuu0123/article/details/4493757


Java为了提供平台无关性，在操作系统之上加入了一层JVM来隔离操作系统特定实现，使所编写的java代码在任何平台都能运行，但是JVM是特定于某一操作系统的





一、当JVM启动时，由三个类加载器对类进行加载：
1.bootstrap classloader
2.extension classloader
3.system classloader



（1）bootstrap classloader［ 引导类加载器］ 是由JVM实现的，不是java.lang.ClassLoader的子类 ，它负责加载Java的核心类，其加载的类由 sun.boot.class.path指定，或者在 执行java命令时使用-Xbootclasspath选项或使用-D选项指定sun.boot.class.path系统属性值



说明：

-Dproperty_name=property_value  指定属性的值；

-Xbootclasspath   改变虚拟机装载缺省系统运行包rt.jar，而从-Xbootclasspath中设定的搜索路径中装载系统运行类



输出加载的核心类库：

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01.public class TestBootstrapClassLoader { 
02.    public static void main(String[] args) { 
03.        URL[] urls=sun.misc.Launcher.getBootstrapClassPath().getURLs(); 
04.        for (int i = 0; i < urls.length; i++) { 
05.          System.out.println(urls[i].toExternalForm()); 
06.        } 
07.    } 
08.} 
public class TestBootstrapClassLoader {
public static void main(String[] args) {
URL[] urls=sun.misc.Launcher.getBootstrapClassPath().getURLs();
for (int i = 0; i < urls.length; i++) {
  System.out.println(urls[i].toExternalForm());
}
}
}

输出结果：

file:/E:/MyEclipse%205.1.1%20GA/jre/lib/rt.jar
file:/E:/MyEclipse%205.1.1%20GA/jre/lib/i18n.jar
file:/E:/MyEclipse%205.1.1%20GA/jre/lib/sunrsasign.jar
file:/E:/MyEclipse%205.1.1%20GA/jre/lib/jsse.jar
file:/E:/MyEclipse%205.1.1%20GA/jre/lib/jce.jar
file:/E:/MyEclipse%205.1.1%20GA/jre/lib/charsets.jar
file:/E:/MyEclipse%205.1.1%20GA/jre/classes



（2） extension classloader［扩展类加载器］ ，它负责加载JRE的扩展目录（JAVA_HOME/jre/lib/ext或者由java.ext.dirs系统属性指定的）中JAR的类包。这为引入除Java核心类以外的新功能提供了一个标准机制。在这个实例上调用方法getParent()总是返回空值null，因为引导加载器bootstrap classloader不是一个真正的ClassLoader实例。

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01.class TestExtensionClassLoader { 
02.    public static void main(String[] args) { 
03.        System.out.println(System.getProperty("java.ext.dirs")); 
04.        ClassLoader extensionClassloader=ClassLoader.getSystemClassLoader().getParent(); 
05.        System.out.println("the parent of extension classloader : "+extensionClassloader.getParent()); 
06.    } 
07.} 
class TestExtensionClassLoader {
public static void main(String[] args) {
System.out.println(System.getProperty("java.ext.dirs"));
ClassLoader extensionClassloader=ClassLoader.getSystemClassLoader().getParent();
System.out.println("the parent of extension classloader : "+extensionClassloader.getParent());
}
}

输出结果：

E:/MyEclipse 5.1.1 GA/jre/lib/ext
the parent of extension classloader : null




以上这段代码也表明了下列父子关系或加载顺序：

bootstrap classloader(因其不是ClassLoader的子类，null)-->extension classloader-->system classloader

这也表明了jvm在加载类的顺序，当加载一个类时（假设其未加载），先找到最顶层的classloader，如果其可以加载这个类（或者已经加载了这个类），则返回这个类；如果其不能加载类（换个说法：在相应路径中搜索不到相应类），则用其子classloader加载，直到这个类被加载或者抛出相应的异常

这个顺序保证了越重要的类，越先加载；因为一个类只被加载一次（cache），所以如java.lang.System这个类，不能被用户替换(因为是按照bootstrap-->extension-->system的顺序，当要加载java.lang.System类时，其首先从bootstrap的搜索路径中找类）





（3）system/application classloader ［系统（应用）类加载器］ ，加载来自-classpath或者java.class.path系统属性或者CLASSPATH操作系统属性所指定的JAR类包和类路径。

a.可以通过静态方法ClassLoader.getSystemClassLoader()找到该类加载器。

b.如果没有特别指定，则用户自定义的任何类加载器都将该类加载器作为它的父加载器。可以从源代码看出这一点：

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01.protected ClassLoader() { 
02.    SecurityManager security = System.getSecurityManager(); 
03.    if (security != null) { 
04.        security.checkCreateClassLoader(); 
05.    } 
06.    this.parent = getSystemClassLoader(); 
07.    initialized = true; 
08.    } 
protected ClassLoader() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
    security.checkCreateClassLoader();
}
this.parent = getSystemClassLoader();
initialized = true;
    }


classpath路径：


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01.class TestSystemClassLoader { 
02.    public static void main(String[] args) { 
03.        System.out.println(System.getProperty("java.class.path")); 
04.    } 
05.} 
class TestSystemClassLoader {
public static void main(String[] args) {
System.out.println(System.getProperty("java.class.path"));
}
}
结果为：

E:/dev/java/eclipse/TestClassLoader/classes;E:/dev/java/lib/commons-logging-1.1.jar



重要：

classloader加载类用的是全盘负责委托机制。

a.全盘负责，即是当一个classloader加载一个Class的时候，这个Class所依赖的和引用的所有Class也由这个classloader负责载入，除非是显式的使用另外一个classloader载入 ；

b.委托机制则是先让parent类加载器 (而不是super，它与parent classloader类不是继承关系)寻找，只有在parent找不到的时候才从自己的类路径中去寻找。此外类加载还采用了cache机制，也就是如果 cache中保存了这个Class就直接返回它，如果没有才从文件中读取和转换成Class，并存入cache，这就是为什么我们修改了Class但是必须重新启动JVM才能生效的原因。

二、ClassLoader加载Class的过程
1.检测此Class是否载入过（即在cache中是否有此Class），如果有到8,如果没有到2
2.如果parent classloader不存在（没有parent，那parent一定是bootstrap classloader了），到4
3.请求parent classloader载入，如果成功到8，不成功到5
4.请求jvm从bootstrap classloader中载入，如果成功到8
5.寻找Class文件（从与此classloader相关的类路径中寻找）。如果找不到则到7.
6.从文件中载入Class，到8.
7.抛出ClassNotFoundException.
8.返回Class.


源代码如下：

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01.ClassLoader类中 
02.protected synchronized Class<?> loadClass(String name, boolean resolve) 
03.    throws ClassNotFoundException 
04.    { 
05.    // First, check if the class has already been loaded  
06.    Class c = findLoadedClass(name); 
07.    if (c == null) { 
08.        try { 
09.        if (parent != null) { 
10.            c = parent.loadClass(name, false); 
11.        } else { 
12.            c = findBootstrapClass0(name); 
13.        } 
14.        } catch (ClassNotFoundException e) { 
15.            // If still not found, then invoke findClass in order  
16.            // to find the class.  
17.            c = findClass(name); 
18.        } 
19.    } 
20.    if (resolve) { 
21.        resolveClass(c); 
22.    } 
23.    return c; 
24.    } 
ClassLoader类中
protected synchronized Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
    {
// First, check if the class has already been loaded
Class c = findLoadedClass(name);
if (c == null) {
    try {
if (parent != null) {
    c = parent.loadClass(name, false);
} else {
    c = findBootstrapClass0(name);
}
    } catch (ClassNotFoundException e) {
        // If still not found, then invoke findClass in order
        // to find the class.
        c = findClass(name);
    }
}
if (resolve) {
    resolveClass(c);
}
return c;
    }

可以看到当在parent链classloader中和bootstrap classloader中都找不到相应的类时，会调用findClass方法，因此ClassLoader的子类可以重写这个方法，定义自己的找到类的方法







以下是一些类的ClassLoader示例：

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01.class TestClassLoader { 
02.    public static void main(String[] args) { 
03.        System.out.println(System.class.getClassLoader()); 
04.        System.out.println(javax.swing.JButton.class.getClassLoader()); 
05.        System.out.println(sun.net.spi.nameservice.dns.DNSNameService.class.getClassLoader()); 
06.        System.out.println(org.apache.commons.logging.Log.class.getClassLoader()); 
07.    } 
08.} 
class TestClassLoader {
public static void main(String[] args) {
System.out.println(System.class.getClassLoader());
System.out.println(javax.swing.JButton.class.getClassLoader());
System.out.println(sun.net.spi.nameservice.dns.DNSNameService.class.getClassLoader());
System.out.println(org.apache.commons.logging.Log.class.getClassLoader());
}
}

输出：

null        其路径在sun.boot.class.path中，由bootstrap classloader加载，所以返回为null（因其不是ClassLoader的子类）

null        其路径在sun.boot.class.path中，由bootstrap classloader加载
sun.misc.Launcher$ExtClassLoader@7259da      路径在java.ext.dirs中，由extension classloader加载
sun.misc.Launcher$AppClassLoader@197d257   由classpath指定，由system classloader加载



查看sun.misc.Launcher的源代码

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01.public Launcher() { 
02.    // Create the extension class loader  
03.    ClassLoader extcl; 
04.    try { 
05.        extcl = ExtClassLoader.getExtClassLoader(); 
06.    } catch (IOException e) { 
07.        throw new InternalError( 
08.        "Could not create extension class loader"); 
09.    } 
10.    // Now create the class loader to use to launch the application  
11.    try { 
12.        loader = AppClassLoader.getAppClassLoader(extcl); 
13.    } catch (IOException e) { 
14.        throw new InternalError( 
15.        "Could not create application class loader"); 
16.    } 
17.    // Also set the context class loader for the primordial thread.  
18.    Thread.currentThread().setContextClassLoader(loader); 
19.        // 其他  
20.} 
public Launcher() {
// Create the extension class loader
ClassLoader extcl;
try {
    extcl = ExtClassLoader.getExtClassLoader();
} catch (IOException e) {
    throw new InternalError(
"Could not create extension class loader");
}
// Now create the class loader to use to launch the application
try {
    loader = AppClassLoader.getAppClassLoader(extcl);
} catch (IOException e) {
    throw new InternalError(
"Could not create application class loader");
}
// Also set the context class loader for the primordial thread.
Thread.currentThread().setContextClassLoader(loader);
        // 其他
}

可以看到是由Launcher这个类初始化ExtClassLoader和AppClassLoader类的

ExtClassLoader无parent，而AppClassLoader的parent为 ExtClassLoader



Launcher的getClassLoader()方法


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01.public ClassLoader getClassLoader() { 
02.    return loader; 
03.    } 
04.只返回AppClassLoader 
public ClassLoader getClassLoader() {
return loader;
    }
只返回AppClassLoader



ExtClassLoader

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01.static class ExtClassLoader extends URLClassLoader { 
02.    private File[] dirs; 
03.    /**
04.     * create an ExtClassLoader. The ExtClassLoader is created
05.     * within a context that limits which files it can read
06.     */ 
07.    public static ExtClassLoader getExtClassLoader() throws IOException 
08.    { 
09.        final File[] dirs = getExtDirs(); 
10.        try { 
11.        // Prior implementations of this doPrivileged() block supplied   
12.        // aa synthesized ACC via a call to the private method  
13.        // ExtClassLoader.getContext().  
14.        return (ExtClassLoader) AccessController.doPrivileged( 
15.             new PrivilegedExceptionAction() { 
16.            public Object run() throws IOException { 
17.                            int len = dirs.length; 
18.                            for (int i = 0; i < len; i++) { 
19.                                MetaIndex.registerDirectory(dirs[i]); 
20.                            } 
21.                            return new ExtClassLoader(dirs); 
22.            } 
23.            }); 
24.        } catch (java.security.PrivilegedActionException e) { 
25.            throw (IOException) e.getException(); 
26.        } 
27.    } 
28....... 
29.private static File[] getExtDirs() { 
30.        String s = System.getProperty("java.ext.dirs"); 
31.        File[] dirs; 
32.        if (s != null) { 
33.        StringTokenizer st =  
34.            new StringTokenizer(s, File.pathSeparator); 
35.        int count = st.countTokens(); 
36.        dirs = new File[count]; 
37.        for (int i = 0; i < count; i++) { 
38.            dirs[i] = new File(st.nextToken()); 
39.        } 
40.        } else { 
41.        dirs = new File[0]; 
42.        } 
43.        return dirs; 
44.    } 
static class ExtClassLoader extends URLClassLoader {
private File[] dirs;
/**
* create an ExtClassLoader. The ExtClassLoader is created
* within a context that limits which files it can read
*/
public static ExtClassLoader getExtClassLoader() throws IOException
{
    final File[] dirs = getExtDirs();
    try {
// Prior implementations of this doPrivileged() block supplied
// aa synthesized ACC via a call to the private method
// ExtClassLoader.getContext().
return (ExtClassLoader) AccessController.doPrivileged(
     new PrivilegedExceptionAction() {
public Object run() throws IOException {
                            int len = dirs.length;
                            for (int i = 0; i < len; i++) {
                                MetaIndex.registerDirectory(dirs[i]);
                            }
                            return new ExtClassLoader(dirs);
}
    });
    } catch (java.security.PrivilegedActionException e) {
    throw (IOException) e.getException();
    }
}
......
private static File[] getExtDirs() {
    String s = System.getProperty("java.ext.dirs");
    File[] dirs;
    if (s != null) {
StringTokenizer st =
    new StringTokenizer(s, File.pathSeparator);
int count = st.countTokens();
dirs = new File[count];
for (int i = 0; i < count; i++) {
    dirs[i] = new File(st.nextToken());
}
    } else {
dirs = new File[0];
    }
    return dirs;
}



AppClassLoader

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01.static class AppClassLoader extends URLClassLoader { 
02.    public static ClassLoader getAppClassLoader(final ClassLoader extcl) 
03.        throws IOException 
04.    { 
05.        final String s = System.getProperty("java.class.path"); 
06.        final File[] path = (s == null) ? new File[0] : getClassPath(s); 
07.        // Note: on bugid 4256530  
08.        // Prior implementations of this doPrivileged() block supplied   
09.        // a rather restrictive ACC via a call to the private method  
10.        // AppClassLoader.getContext(). This proved overly restrictive  
11.        // when loading  classes. Specifically it prevent  
12.        // accessClassInPackage.sun.* grants from being honored.  
13.        //  
14.        return (AppClassLoader)  
15.        AccessController.doPrivileged(new PrivilegedAction() { 
16.        public Object run() { 
17.            URL[] urls = 
18.            (s == null) ? new URL[0] : pathToURLs(path); 
19.            return new AppClassLoader(urls, extcl); 
20.        } 
21.        }); 
22.    } 
23....... 
static class AppClassLoader extends URLClassLoader {
public static ClassLoader getAppClassLoader(final ClassLoader extcl)
    throws IOException
{
    final String s = System.getProperty("java.class.path");
    final File[] path = (s == null) ? new File[0] : getClassPath(s);
    // Note: on bugid 4256530
    // Prior implementations of this doPrivileged() block supplied
    // a rather restrictive ACC via a call to the private method
    // AppClassLoader.getContext(). This proved overly restrictive
    // when loading  classes. Specifically it prevent
    // accessClassInPackage.sun.* grants from being honored.
    //
    return (AppClassLoader)
AccessController.doPrivileged(new PrivilegedAction() {
public Object run() {
    URL[] urls =
(s == null) ? new URL[0] : pathToURLs(path);
    return new AppClassLoader(urls, extcl);
}
    });
}
......




Launcher类的getBootstrapClassPath()方法

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01.public static URLClassPath getBootstrapClassPath() { 
02.    String prop = (String)AccessController.doPrivileged(new GetPropertyAction("sun.boot.class.path")); 
03.    URL[] urls; 
04.    if (prop != null) { 
05.        final String path = prop; 
06.        urls = (URL[])AccessController.doPrivileged( 
07.        new PrivilegedAction() { 
08.            public Object run() { 
09.                        File[] classPath = getClassPath(path); 
10.                        int len = classPath.length; 
11.                        Set seenDirs = new HashSet(); 
12.                        for (int i = 0; i < len; i++) { 
13.                            File curEntry = classPath[i]; 
14.                            // Negative test used to properly handle  
15.                            // nonexistent jars on boot class path  
16.                            if (!curEntry.isDirectory()) { 
17.                                curEntry = curEntry.getParentFile(); 
18.                            } 
19.                            if (curEntry != null && seenDirs.add(curEntry)) { 
20.                                MetaIndex.registerDirectory(curEntry); 
21.                            } 
22.                        } 
23.                        return pathToURLs(classPath); 
24.            } 
25.        } 
26.        ); 
27.    } else { 
28.        urls = new URL[0]; 
29.    } 
30.    return new URLClassPath(urls, factory); 
31.    } 
public static URLClassPath getBootstrapClassPath() {
String prop = (String)AccessController.doPrivileged(new GetPropertyAction("sun.boot.class.path"));
URL[] urls;
if (prop != null) {
    final String path = prop;
    urls = (URL[])AccessController.doPrivileged(
new PrivilegedAction() {
    public Object run() {
                        File[] classPath = getClassPath(path);
                        int len = classPath.length;
                        Set seenDirs = new HashSet();
                        for (int i = 0; i < len; i++) {
                            File curEntry = classPath[i];
                            // Negative test used to properly handle
                            // nonexistent jars on boot class path
                            if (!curEntry.isDirectory()) {
                                curEntry = curEntry.getParentFile();
                            }
                            if (curEntry != null && seenDirs.add(curEntry)) {
                                MetaIndex.registerDirectory(curEntry);
                            }
                        }
                        return pathToURLs(classPath);
    }
}
    );
} else {
    urls = new URL[0];
}
return new URLClassPath(urls, factory);
    }

从以上代码中可以看到bootstrap classloader使用sun.boot.class.path来加载类，extension classloader使用java.ext.dirs来加载类，而system classloader使用java.class.path来加载类



运行下列程序：

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01.class Test1 { 
02.    public static void main(String[] args) { 
03.        System.out.println(sun.misc.Launcher.getLauncher().getClass().getClassLoader()); 
04.    } 
05.} 
class Test1 {
public static void main(String[] args) {
System.out.println(sun.misc.Launcher.getLauncher().getClass().getClassLoader());
}
}

结果为：

null

表明Launcher是由bootstrap classloader来加载的



三、关于Context ClassLoader
在上面Launcher的构造函数中有这么一句： Thread.currentThread().setContextClassLoader(loader); 这句是设置当前线程的classloader，默认是使用的AppClassLoader

这个有什么作用呢?

当线程需要用到某个类，contextClassLoader被请求来载入该类

注意：

（1）Class.forName(String name)载入的是在系统中已经加载入sun.boot.class.path、 java.ext.dirs、 java.class.path路径中的类，而在这几个路径中未加入的类不能载入（报异常）

（2）Class.forName(String name, boolean initialize, ClassLoader loader)可以载入上述三个路径中没有的类，只要指定你的classloader即可


（3）利用ClassLoader可以载入在上述三个路径中没有的类

示例：

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01.自定义ClassLoader，从c根目录中读class 
02.package Test; 
03.public class TestClassLoader extends ClassLoader { 
04.        protected Map<String, Class> cache = new HashMap<String, Class>(); 
05.     
06.    public TestClassLoader() { 
07.        super(); 
08.    } 
09.     
10.    protected Class<?> findClass(String name) throws ClassNotFoundException { 
11.                if (cache.get(name) != null) { 
12.            return cache.get(name); 
13.        } 
14.        try { 
15.            String tname = name.replace('.', '/'); 
16.            File file = new File("c://" + tname + ".class"); 
17.            FileInputStream in = new FileInputStream(file); 
18.            BufferedInputStream bufIn = new BufferedInputStream(in); 
19.            ByteArrayOutputStream byteOut = new ByteArrayOutputStream(); 
20.            BufferedOutputStream bufOut = new BufferedOutputStream(byteOut); 
21.            byte[] buffer = new byte[4096]; 
22.            int len = -1; 
23.            while((len = bufIn.read(buffer)) != -1) { 
24.                bufOut.write(buffer, 0, len); 
25.            } 
26.            bufOut.flush(); 
27.            byteOut.flush(); 
28.            byte[] data = byteOut.toByteArray(); 
29.            Class cls = defineClass(name, data, 0, data.length); 
30.            cache.put(name, cls); 
31.            return cls;     } catch (IOException e) { 
32.            e.printStackTrace(); 
33.        } 
34.        return null; 
35.    } 
36.} 
37.class TestLoader { 
38.    public static void main(String[] args) throws InstantiationException, IllegalAccessException, ClassNotFoundException { 
39.        ClassLoader cl = Thread.currentThread().getContextClassLoader(); 
40.        System.out.println(Thread.currentThread().toString() + "  " + cl.toString()); 
41.        new Thread() { 
42.            public void run() { 
43.                ClassLoader cl = Thread.currentThread().getContextClassLoader(); 
44.                System.out.println(Thread.currentThread().toString() + "  " + cl.toString()); 
45.                 
46.                TestClassLoader loader = new TestClassLoader(); 
47.                Thread.currentThread().setContextClassLoader(loader); 
48.                new Thread() { 
49.                    public void run() { 
50.                        try { 
51.                            ClassLoader cl = Thread.currentThread().getContextClassLoader(); 
52.                            System.out.println(Thread.currentThread().toString() + "  " + cl.toString()); 
53.    //ClassLoader                         
54.          AbstractTestA test = (AbstractTestA) cl.loadClass("Test.TestAImp1").newInstance(); 
55.        //Class.forName  
56.    //AbstractTestA test = (AbstractTestA) Class.forName("Test.TestAImp1", true, cl).newInstance();                     test.test();  
57.                        } catch (InstantiationException e) { 
58.                            // TODO Auto-generated catch block  
59.                            e.printStackTrace(); 
60.                        } catch (IllegalAccessException e) { 
61.                            // TODO Auto-generated catch block  
62.                            e.printStackTrace(); 
63.                        } catch (ClassNotFoundException e) { 
64.                            // TODO Auto-generated catch block  
65.                            e.printStackTrace(); 
66.                        } 
67.                    } 
68.                }.start(); 
69.            } 
70.        }.start(); 
71.    } 
72.} 
73.//抽象类  
74.package Test; 
75.public abstract class AbstractTestA { 
76.    public String a; 
77.    public void print() { 
78.        System.out.println("a=" + a); 
79.    } 
80.     
81.    abstract public void test(); 
82.} 
83.//编译后放入c根目录  
84.package Test; 
85.public class TestAImp1 extends AbstractTestA { 
86.    public void test() { 
87.        System.out.println(Thread.currentThread().toString()); 
88.        System.out.println("Test"); 
89.    } 
90.} 
自定义ClassLoader，从c根目录中读class
package Test;
public class TestClassLoader extends ClassLoader {
        protected Map<String, Class> cache = new HashMap<String, Class>();

public TestClassLoader() {
super();
}

protected Class<?> findClass(String name) throws ClassNotFoundException {
                if (cache.get(name) != null) {
return cache.get(name);
}
try {
String tname = name.replace('.', '/');
File file = new File("c://" + tname + ".class");
FileInputStream in = new FileInputStream(file);
BufferedInputStream bufIn = new BufferedInputStream(in);
ByteArrayOutputStream byteOut = new ByteArrayOutputStream();
BufferedOutputStream bufOut = new BufferedOutputStream(byteOut);
byte[] buffer = new byte[4096];
int len = -1;
while((len = bufIn.read(buffer)) != -1) {
bufOut.write(buffer, 0, len);
}
bufOut.flush();
byteOut.flush();
byte[] data = byteOut.toByteArray();
Class cls = defineClass(name, data, 0, data.length);
cache.put(name, cls);
return cls; } catch (IOException e) {
e.printStackTrace();
}
return null;
}
}
class TestLoader {
public static void main(String[] args) throws InstantiationException, IllegalAccessException, ClassNotFoundException {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
System.out.println(Thread.currentThread().toString() + "  " + cl.toString());
new Thread() {
public void run() {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
System.out.println(Thread.currentThread().toString() + "  " + cl.toString());

TestClassLoader loader = new TestClassLoader();
Thread.currentThread().setContextClassLoader(loader);
new Thread() {
public void run() {
try {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
System.out.println(Thread.currentThread().toString() + "  " + cl.toString());
//ClassLoader
          AbstractTestA test = (AbstractTestA) cl.loadClass("Test.TestAImp1").newInstance();
        //Class.forName
//AbstractTestA test = (AbstractTestA) Class.forName("Test.TestAImp1", true, cl).newInstance(); test.test();
} catch (InstantiationException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalAccessException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}.start();
}
}.start();
}
}
//抽象类
package Test;
public abstract class AbstractTestA {
public String a;
public void print() {
System.out.println("a=" + a);
}

abstract public void test();
}
//编译后放入c根目录
package Test;
public class TestAImp1 extends AbstractTestA {
public void test() {
System.out.println(Thread.currentThread().toString());
System.out.println("Test");
}
}

运行结果为：

Thread[main,5,main]  sun.misc.Launcher$AppClassLoader@197d257
Thread[Thread-0,5,main]  sun.misc.Launcher$AppClassLoader@197d257
Thread[Thread-1,5,main]  Test.TestClassLoader@1004901
Thread[Thread-1,5,main]
Test

这也表明了线程间ContextClassLoader的继承性：

（1）main线程默认的 ContextClassLoader为AppClassLoader

（2）新启动线程从原线程处继承 ContextClassLoader



四、关于ClassLoader和Package
其中：String(int offset, int len, char[] arr)为包访问权限

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01.package java.lang; 
02.public class TestPackage { 
03.    public static void main(String[] args) { 
04.        char[] c = "1234567890".toCharArray(); 
05.        String s = new String(0, 10, c); 
06.    } 
07.} 
package java.lang;
public class TestPackage {
public static void main(String[] args) {
char[] c = "1234567890".toCharArray();
String s = new String(0, 10, c);
}
}

此代码可以编译通过，但是运行时出现下列错误：

java.lang.SecurityException: Prohibited package name: java.lang
at java.lang.ClassLoader.preDefineClass(Unknown Source)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$100(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)



这表明：

Java语言规定，在同一个包中的class，如果没有修饰符，默认为Package权限，包内的class都可以访问。但是这还不够准确。确切的说，只有由同一个ClassLoader装载的class才具有以上的Package权限。比如Bootstrap classloader装载了java.lang.String，AppClassLoader装载了我们自己写的java.lang.TestPackage，它们不能互相访问对方具有Package权限的方法。这样就阻止了恶意代码访问核心类的Package权限方法。



五、关于两个ClassLoader载入同一个类
注意：由两个不同的ClassLoader载入的同一个类，其是不同类型的，因此如果进行赋值会报ClassCastException

示例：

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01.package Test; 
02.import java.io.*; 
03.import java.util.*; 
04.public class TestClassLoader extends ClassLoader { 
05.     
06.    protected Map<String, Class> cache = new HashMap<String, Class>(); 
07.     
08.    public TestClassLoader() { 
09.        super(); 
10.    } 
11.     
12.    protected Class<?> findClass(String name) throws ClassNotFoundException { 
13.        if (cache.get(name) != null) { 
14.            return cache.get(name); 
15.        } 
16.        try { 
17.            String tname = name.replace('.', '/'); 
18.            File file = new File("c://" + tname + ".class"); 
19.            FileInputStream in = new FileInputStream(file); 
20.            BufferedInputStream bufIn = new BufferedInputStream(in); 
21.            ByteArrayOutputStream byteOut = new ByteArrayOutputStream(); 
22.            BufferedOutputStream bufOut = new BufferedOutputStream(byteOut); 
23.            byte[] buffer = new byte[4096]; 
24.            int len = -1; 
25.            while((len = bufIn.read(buffer)) != -1) { 
26.                bufOut.write(buffer, 0, len); 
27.            } 
28.            bufOut.flush(); 
29.            byteOut.flush(); 
30.            byte[] data = byteOut.toByteArray(); 
31.            Class cls = defineClass(name, data, 0, data.length); 
32.            cache.put(name, cls); 
33.            return cls; 
34.        } catch (IOException e) { 
35.            e.printStackTrace(); 
36.        } 
37.        return null; 
38.    } 
39.} 
40.class TestLoaderA { 
41.    public static void main(String[] args) throws InstantiationException, IllegalAccessException, ClassNotFoundException { 
42.        TestClassLoader loader = new TestClassLoader(); 
43.        System.out.println(TestAImp1.class.getClassLoader()); 
44.        System.out.println(loader.findClass("Test.TestAImp1").getClassLoader()); 
45.        TestAImp1 test = (TestAImp1)loader.findClass("Test.TestAImp1").newInstance(); 
46.        test.test(); 
47.         
48.    } 
49.} 
50.public class TestAImp1 extends AbstractTestA { 
51.    public void test() { 
52.        System.out.println(Thread.currentThread().toString()); 
53.        System.out.println("Test"); 
54.    } 
55.} 
package Test;
import java.io.*;
import java.util.*;
public class TestClassLoader extends ClassLoader {

protected Map<String, Class> cache = new HashMap<String, Class>();

public TestClassLoader() {
super();
}

protected Class<?> findClass(String name) throws ClassNotFoundException {
if (cache.get(name) != null) {
return cache.get(name);
}
try {
String tname = name.replace('.', '/');
File file = new File("c://" + tname + ".class");
FileInputStream in = new FileInputStream(file);
BufferedInputStream bufIn = new BufferedInputStream(in);
ByteArrayOutputStream byteOut = new ByteArrayOutputStream();
BufferedOutputStream bufOut = new BufferedOutputStream(byteOut);
byte[] buffer = new byte[4096];
int len = -1;
while((len = bufIn.read(buffer)) != -1) {
bufOut.write(buffer, 0, len);
}
bufOut.flush();
byteOut.flush();
byte[] data = byteOut.toByteArray();
Class cls = defineClass(name, data, 0, data.length);
cache.put(name, cls);
return cls;
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
}
class TestLoaderA {
public static void main(String[] args) throws InstantiationException, IllegalAccessException, ClassNotFoundException {
TestClassLoader loader = new TestClassLoader();
System.out.println(TestAImp1.class.getClassLoader());
System.out.println(loader.findClass("Test.TestAImp1").getClassLoader());
TestAImp1 test = (TestAImp1)loader.findClass("Test.TestAImp1").newInstance();
test.test();

}
}
public class TestAImp1 extends AbstractTestA {
public void test() {
System.out.println(Thread.currentThread().toString());
System.out.println("Test");
}
}

运行以上代码结果：

sun.misc.Launcher$AppClassLoader@197d257
Test.TestClassLoader@1b90b39
Exception in thread "main" java.lang.ClassCastException: Test.TestAImp1
at Test.TestLoaderA.main(TestClassLoader.java:84)