一.类加载器分为四类：

        1.启动类加载器（Bootstrap classLoader）：负责加载支撑JVM运行的位于JRE的lib目录下的核心类库，比如 rt.jar、charsets.jar等

        2.扩展类加载器（ExtClassLoader）：负责加载支撑JVM运行的位于JRE的lib目录下的ext扩展目录中 的JAR类包

        3.应用程序类加载器（AppClassLoader）：负责加载ClassPath路径下的类包，主要就是加载你自己写（即项目对应路径下的） 的那些类

        4.自定义加载器：负责加载用户自定义路径下的类包

二.加载机制介绍：（双亲委派）

        双亲委派机制整体流程图示：

 源码分析：

protected Class<?> loadClass(String name, boolean resolve)
    throws ClassNotFoundException
{
    synchronized (getClassLoadingLock(name)) {
        // First, check if the class has already been loaded
        //判断该类是否已经被加载
        Class<?> c = findLoadedClass(name);
        if (c == null) {
            long t0 = System.nanoTime();
            try {
                if (parent != null) {
                     //父类不为空交给父类去加载   
                    c = parent.loadClass(name, false);
                } else {
                        //父类为空则说明，说明是启动类加载器，
                     //因为底层是c说编写，java是找不到该类的
                    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();
                //如果加载不到，怎调用其子类的方法去找
                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;
    }
}

其中 ext和app都是classLoader的子类不存在真实的继承关系，都是Launcher的内部类，通过Launcher类的初始的时候设置伪父类的关系：

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);
    }

}

创建这两个加载器的时候都会去调用classLoader的构造器，将其父类设值

protected ClassLoader(ClassLoader parent) {
    this(checkCreateClassLoader(), parent);
}

当我们自定义类加载器的时候，只需要重写其findClass方法即可，如果要打破双亲委派机制，重新loadClass方法，其默认的构造器是将appClassLoader。

protected ClassLoader() {
    this(checkCreateClassLoader(), getSystemClassLoader());
}

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

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;
    }
}