jvm复习： ClassLoder.getSystemClassLoader的执行流程

其中scl为本类中声明的变量，用来保存系统类加载器，

private static ClassLoader scl;

然后通过在getSystemClassLoader方法时，通过初始化方法来给其赋值并返回

public static ClassLoader getSystemClassLoader() {
        initSystemClassLoader();
        if (scl == null) {
            return null;
        }
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkClassLoaderPermission(scl, Reflection.getCallerClass());
        }
        return scl;
    }

initSystemClassLoader()用来完成scl的赋值：

private static synchronized void initSystemClassLoader() {
        if (!sclSet) {
            if (scl != null)
                throw new IllegalStateException("recursive invocation");
            sun.misc.Launcher l = sun.misc.Launcher.getLauncher();
            if (l != null) {
                Throwable oops = null;
                scl = l.getClassLoader();
                try {
                    scl = AccessController.doPrivileged(
                        new SystemClassLoaderAction(scl));
                } catch (PrivilegedActionException pae) {
                    oops = pae.getCause();
                    if (oops instanceof InvocationTargetException) {
                        oops = oops.getCause();
                    }
                }
                if (oops != null) {
                    if (oops instanceof Error) {
                        throw (Error) oops;
                    } else {
                        // wrap the exception
                        throw new Error(oops);
                    }
                }
            }
            sclSet = true;
        }
    }

其中sun.misc.Launcher l = sun.misc.Launcher.getLauncher();定义了一个Launcher类，该类有个成员变量classLoader来保存系统类加载器，其构造方法给classLoader赋值。
通过scl = l.getClassLoader();拿到了Launcher类中的classLoader成员变量