Java类加载器ClassLoader源码层面分析讲解

public class Launcher {
	// 成员 ClassLoader 类加载器，用来存储 应用程序类加载器，
	// 此加载器与线程绑定，用作线程的上下文类加载器。
	private ClassLoader loader;  
	// 两个静态内部类
	static class AppClassLoader extends URLClassLoader {...}
	static class ExtClassLoader extends URLClassLoader {...}
	    // 构造器
	    public Launcher() {
        Launcher.ExtClassLoader var1;
        try {
            var1 = Launcher.ExtClassLoader.getExtClassLoader();
        } catch (IOException var10) {
            throw new InternalError("Could not create extension class loader", var10);
        }
        try {
            this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
        } catch (IOException var9) {
            throw new InternalError("Could not create application class loader", var9);
        }
        Thread.currentThread().setContextClassLoader(this.loader);
        String var2 = System.getProperty("java.security.manager");
        if (var2 != null) {
            SecurityManager var3 = null;
            if (!"".equals(var2) && !"default".equals(var2)) {
                try {
                    var3 = (SecurityManager)this.loader.loadClass(var2).newInstance();
                } catch (IllegalAccessException var5) {
                } catch (InstantiationException var6) {
                } catch (ClassNotFoundException var7) {
                } catch (ClassCastException var8) {
                }
            } else {
                var3 = new SecurityManager();
            }
            if (var3 == null) {
                throw new InternalError("Could not create SecurityManager: " + var2);
            }
            System.setSecurityManager(var3);
        }
    }
}

 static class AppClassLoader extends URLClassLoader { 
        public Class<?> loadClass(String var1, boolean var2) throws ClassNotFoundException {
            // ...
            // 调用了一个 native 本地方法 knownToNotExist()，去查找该类的加载记录
            if (this.ucp.knownToNotExist(var1)) {        // 如果有该类加载记录
                Class var5 = this.findLoadedClass(var1); // 直接去已经加载的类中找
                if (var5 != null) {
                    if (var2) {
                        this.resolveClass(var5);
                    }
                    return var5;
                } else {                                 // 如果没找到报异常
                    throw new ClassNotFoundException(var1);
                }
            } else {                                     // 如果没有该类的加载记录
                return super.loadClass(var1, var2);      // 调用父类 ClassLoader 的 loadClass()
            }
        }
}

static class ExtClassLoader extends URLClassLoader {
	//...
}

 /** 允许直接调用的是这个根据 类名加载类 的方法， */
 public Class<?> loadClass(String name) throws ClassNotFoundException {
        return loadClass(name, false); // 内部调用的是类加载方法默认不对类进行解析
    }

 	protected Class<?> loadClass(String name, boolean resolve)  throws ClassNotFoundException {
        synchronized (getClassLoadingLock(name)) { // 加载一个类的时候，锁住对应的类加载器对象
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);    // 查询已加载的类中是否有该类存在 
            if (c == null) {
                long t0 = System.nanoTime();       // 记录开始时间
                try {
                    if (parent != null) {          // 非顶层类加载器
                        c = parent.loadClass(name, false);  // 调用父类加载器进行加载
                    } else {
                        c = findBootstrapClassOrNull(name); // 调用启动类加载器加载
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }
                if (c == null) { // 如果在 父类加载器 或 启动类加载器 没有找到需要加载的类
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();  // 记录结束时间
                    c = findClass(name);          // 再由当前加载器加载
                    // this is the defining class loader; record the stats
                    // 性能计数器
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c); // 对类进行解析
            }
            return c;
        }
    }

    protected Class<?> findClass(String name) throws ClassNotFoundException {
        throw new ClassNotFoundException(name);
    }

    // Maps class name to the corresponding lock object when the current
    // class loader is parallel capable.
    // Note: VM also uses this field to decide if the current class loader
    // is parallel capable and the appropriate lock object for class loading.
    private final ConcurrentHashMap<String, Object> parallelLockMap;

    protected Object getClassLoadingLock(String className) {
        Object lock = this; // 这个锁就是类加载器对象本身！
        if (parallelLockMap != null) {
            Object newLock = new Object(); 
            lock = parallelLockMap.putIfAbsent(className, newLock); // 放入集合里存储
            if (lock == null) { // 如果没有对应的类加载器对象
                lock = newLock; // 创建一个新的类加载器
            }
        }
        return lock; // 返回类加载器对象
    }

	 /**
     * Returns a class loaded by the bootstrap class loader;
     * or return null if not found.
     */
    private Class<?> findBootstrapClassOrNull(String name)
    {
        if (!checkName(name)) return null;
        return findBootstrapClass(name);
    }
    // 返回 BootstrapClassLoader 启动类加载器加载的类。
    private native Class<?> findBootstrapClass(String name);