Tomcat ClassLoader打破双亲委派源码解析

protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // 本类加载器从缓存里面拿取已经加载过的ClassLoader
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    // 父类为BootStrapClassLoader的classLoader 的parent变量为空
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }
                ..........
    }

@Override
    public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
        synchronized (getClassLoadingLock(name)) {
           ................
            // 从缓存里面取，不影响加载顺序，请着重忽略
            clazz = findLoadedClass0(name);
            ..........
            clazz = findLoadedClass(name);
            .........
           /**
     * The bootstrap class loader used to load the JavaSE classes. In some
     * implementations this class loader is always <code>null</code> and in
     * those cases {@link ClassLoader#getParent()} will be called recursively on
     * the system class loader and the last non-null result used.
     */
            // 注意看注释，javaseLoader是bootstrap加载的，所以javaseLoader是扩展类加载器
            ClassLoader javaseLoader = getJavaseClassLoader();
            clazz = javaseLoader.loadClass(name);
            boolean delegateLoad = delegate || filter(name, true);
            // (2) Search local repositories
            if (log.isDebugEnabled()) {
                log.debug("  Searching local repositories");
            }
            try {
            // 重写了findClass方法，从项目中的lib和class里加载
                clazz = findClass(name);
                if (clazz != null) {
                    if (log.isDebugEnabled()) {
                        log.debug("  Loading class from local repository");
                    }
                    if (resolve) {
                        resolveClass(clazz);
                    }
                    return clazz;
                }
            } catch (ClassNotFoundException e) {
                // Ignore
            }
            // (3) Delegate to parent unconditionally
            if (!delegateLoad) {
                if (log.isDebugEnabled()) {
                    log.debug("  Delegating to parent classloader at end: " + parent);
                }
                try {
//  继承于SystemClassLoader该类，所以这个parent是SystemClassLoader，
                    clazz = Class.forName(name, false, parent);
                    if (clazz != null) {
                        if (log.isDebugEnabled()) {
                            log.debug("  Loading class from parent");
                        }
                        if (resolve) {
                            resolveClass(clazz);
                        }
                        return clazz;
                    }
                } catch (ClassNotFoundException e) {
                    // Ignore
                }
            }
        }
        throw new ClassNotFoundException(name);
    }