初识类加载器2021/6/9

加载.class文件大致可以分为3个步骤：

1. 检查是否已经加载，有就直接返回，避免重复加载
2. 当前缓存中确实没有该类，那么遵循父优先加载机制，加载.class文件
3. 上面两步都失败了，调用findClass()方法加载

 protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // 首先，检查是否已经加载该类
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    // 如果尚未加载，则遵循父优先的等级加载机制（所谓双亲委派机制）
                    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
                }

                if (c == null) {
                    // 模板方法模式：如果还是没有加载成功，调用findClass()
                    long t1 = System.nanoTime();
                    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;
        }
    }

    // 子类应该重写该方法
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        throw new ClassNotFoundException(name);
    }