那些有趣的代码(二)--偏不听父母话的 Tomcat 类加载器

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

public abstract class ClassLoader {
      // The parent class loader for delegation
      // Note: VM hardcoded the offset of this field, thus all new fields
      // must be added *after* it.
    private final ClassLoader parent;

    protected Class<?> loadClass(String name, boolean resolve)
          throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                    // 尝试使用 父辈的 loadClass 方法
                        c = parent.loadClass(name, false);
                    } else {
                    // 如果没有 父辈的 classLoader 就使用 bootstrap classLoader
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    // 父辈没法加载这个 class，就自己尝试加载
                    c = findClass(name);
                    // this is the defining class loader; record the stats
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
                return c;
        }
    }

    // 根据类名 寻找 class。我们在之前我们讲过，不通过的 classLoader 加载的 class 的位置不同。
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        return defineClass(name, res);
    }

}

    @Override
    public Class<?> findClass(String name) throws ClassNotFoundException {
        // Ask our superclass to locate this class, if possible
        // (throws ClassNotFoundException if it is not found)
        Class<?> clazz = null;

        // 先在自己的 Web 应用目录下查找 class
        clazz = findClassInternal(name);

        // 找不到 在交由父类来处理
        if ((clazz == null) && hasExternalRepositories) {  
            clazz = super.findClass(name);
        }
        if (clazz == null) {
             throw new ClassNotFoundException(name);
        }
        return clazz;
    }

    public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
        synchronized (getClassLoadingLock(name)) {
            Class<?> clazz = null;
            //1. 先在本地cache查找该类是否已经加载过
            clazz = findLoadedClass0(name);
            if (clazz != null) {
                if (resolve)
                    resolveClass(clazz);
                return clazz;
            }
            //2. 从系统类加载器的cache中查找是否加载过
            clazz = findLoadedClass(name);
            if (clazz != null) {
                if (resolve)
                    resolveClass(clazz);
                return clazz;
            }
           // 3. 尝试用ExtClassLoader类加载器类加载
            ClassLoader javaseLoader = getJavaseClassLoader();
            try {
                clazz = javaseLoader.loadClass(name);
                if (clazz != null) {
                    if (resolve)
                        resolveClass(clazz);
                    return clazz;
                }
            } catch (ClassNotFoundException e) {
                // Ignore
            }
            // 4. 尝试在本地目录搜索class并加载
            try {
                clazz = findClass(name);
                if (clazz != null) {
                    if (resolve)
                        resolveClass(clazz);
                    return clazz;
                }
            } catch (ClassNotFoundException e) {
                // Ignore
            }
            // 5. 尝试用系统类加载器(也就是AppClassLoader)来加载
                try {
                    clazz = Class.forName(name, false, parent);
                    if (clazz != null) {
                        if (resolve)
                            resolveClass(clazz);
                        return clazz;
                    }
                } catch (ClassNotFoundException e) {
                    // Ignore
                }
           }
        //6. 上述过程都加载失败，抛出异常
        throw new ClassNotFoundException(name);
    }