双亲委派的优点：
如果有人想要篡改程序的类实现，在这种机制下是无效的，因为这些系统的类已经被BootStrapClassLoader加载过了，不会再次加载，从一定程度上防止了危险代码的植入。

java文件编译完全会生成 .class文件, .class文件就是通过类加载器ClassLoader加载的。ClassLoader在加载过程中会用双亲委派机制加载 .class文件。

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CustomClassLoader ( 自己搞的，可以不用，用appClassLoader基本够用)
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AppClassLoader (应用程序类加载器，appclassloader会加载*Java环境变量 CLASSPATH所指定的路径下的类库 * ，而CLASSPATH所指定的路径可以通过 System.getProperty("java.class.path") 获取，该变量可被覆盖，可以使用 -cp 参数)
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ExtClassLoader（扩展类加载器，会加载 （ $ JAVA_HOME/jre/lib/ext）下的类库）

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BootStrapClassLoader(启动类加载器，JVM启动时创建，用于加载 /JRE/LIB 下面的类库)
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ClassLoader的双亲委派机制：
1.当appclassloader加载一个class时，会先把类加载请求委派给父类的加载器ExtClassLoader去完成加载；
2.当ExtClassLoader加载一个class时，会先把类加载请求委派给父类的加载器BootStrapClassLoader去完成加载；
3.如果BootStrapClassLoader加载失败，会使用ExtClassLoader来尝试加载；
4.如果ExtClassLoader加载失败，会使用AppClassLoader来 加载，如果AppClassLoader也加载失败，则会抛出异常ClassNotFoundException

  /**
     * Loads the class with the specified <a href="#name">binary name</a>.  The
     * default implementation of this method searches for classes in the
     * following order:
     *
     * <ol>
     *
     *   <li><p> Invoke {@link #findLoadedClass(String)} to check if the class
     *   has already been loaded.  </p></li>
     *
     *   <li><p> Invoke the {@link #loadClass(String) <tt>loadClass</tt>} method
     *   on the parent class loader.  If the parent is <tt>null</tt> the class
     *   loader built-in to the virtual machine is used, instead.  </p></li>
     *
     *   <li><p> Invoke the {@link #findClass(String)} method to find the
     *   class.  </p></li>
     *
     * </ol>
     *
     * <p> If the class was found using the above steps, and the
     * <tt>resolve</tt> flag is true, this method will then invoke the {@link
     * #resolveClass(Class)} method on the resulting <tt>Class</tt> object.
     *
     * <p> Subclasses of <tt>ClassLoader</tt> are encouraged to override {@link
     * #findClass(String)}, rather than this method.  </p>
     *
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @param  resolve
     *         If <tt>true</tt> then resolve the class
     *
     * @return  The resulting <tt>Class</tt> object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     */
    // Android-removed: Remove references to getClassLoadingLock
    //                   Remove perf counters.
    //
    // <p> Unless overridden, this method synchronizes on the result of
    // {@link #getClassLoadingLock <tt>getClassLoadingLock</tt>} method
    // during the entire class loading process.
    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                try {
                    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) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    c = findClass(name);
                }
            }
            return c;
    }

打破双亲委派机制：
https://blog.csdn.net/cy973071263/article/details/104129163