<p>单例模式相信大家都知道，用过的人不在少数。之前写过一篇博文《singleton模式四种线程安全的实现》（参见：http://blog.csdn.NET/u013256816/article/details/50427061），讲诉了单例模式的四种写法，并指出占位符模式的写法比较ok，详见如下：</p>    <pre>  <code class="language-java">package com.effective.singleton;      public class Elvis    {        private static boolean flag = false;          private Elvis(){        }          private  static class SingletonHolder{            private static final Elvis INSTANCE = new Elvis();        }          public static Elvis getInstance()        {            return SingletonHolder.INSTANCE;        }          public void doSomethingElse()        {          }    }</code></pre>    <p>但这都是基于一个条件：确保不会通过反射机制调用私有的构造器。</p>    <p>这里举个例子，通过JAVA的反射机制来“攻击”单例模式：</p>    <pre>  <code class="language-java">package com.effective.singleton;      import java.lang.reflect.Constructor;    import java.lang.reflect.InvocationTargetException;      public class ElvisReflectAttack    {          public static void main(String[] args) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException        {            Class<?> classType = Elvis.class;              Constructor<?> c = classType.getDeclaredConstructor(null);            c.setAccessible(true);            Elvis e1 = (Elvis)c.newInstance();            Elvis e2 = Elvis.getInstance();            System.out.println(e1==e2);        }      }</code></pre>    <p>运行结果：false</p>    <p>可以看到，通过反射获取构造函数，然后调用setAccessible(true)就可以调用私有的构造函数，所有e1和e2是两个不同的对象。</p>    <p>如果要抵御这种攻击，可以修改构造器，让它在被要求创建第二个实例的时候抛出异常。</p>    <p>经修改后：</p>    <pre>  <code class="language-java">package com.effective.singleton;      public class ElvisModified    {        private static boolean flag = false;          private ElvisModified(){            synchronized(ElvisModified.class)            {                if(flag == false)                {                    flag = !flag;                }                else                {                    throw new RuntimeException("单例模式被侵犯！");                }            }        }          private  static class SingletonHolder{            private static final ElvisModified INSTANCE = new ElvisModified();        }          public static ElvisModified getInstance()        {            return SingletonHolder.INSTANCE;        }          public void doSomethingElse()        {          }    }</code></pre>    <p>测试代码：</p>    <pre>  <code class="language-java">package com.effective.singleton;      import java.lang.reflect.Constructor;      public class ElvisModifiedReflectAttack    {          public static void main(String[] args)        {            try            {                Class<ElvisModified> classType = ElvisModified.class;                  Constructor<ElvisModified> c = classType.getDeclaredConstructor(null);                c.setAccessible(true);                ElvisModified e1 = (ElvisModified)c.newInstance();                ElvisModified e2 = ElvisModified.getInstance();                System.out.println(e1==e2);            }            catch (Exception e)            {                e.printStackTrace();            }        }    }</code></pre>    <p>运行结果：</p>    <pre>  <code class="language-java">Exception in thread "main" java.lang.ExceptionInInitializerError        at com.effective.singleton.ElvisModified.getInstance(ElvisModified.java:27)        at com.effective.singleton.ElvisModifiedReflectAttack.main(ElvisModifiedReflectAttack.java:17)    Caused by: java.lang.RuntimeException: 单例模式被侵犯！        at com.effective.singleton.ElvisModified.<init>(ElvisModified.java:16)        at com.effective.singleton.ElvisModified.<init>(ElvisModified.java:7)        at com.effective.singleton.ElvisModified$SingletonHolder.<clinit>(ElvisModified.java:22)        ... 2 more</code></pre>    <p>可以看到，成功的阻止了单例模式被破坏。</p>    <p>从JDK1.5开始，实现Singleton还有新的写法，只需编写一个包含单个元素的枚举类型。推荐写法：</p>    <pre>  <code class="language-java">package com.effective.singleton;      public enum SingletonClass    {        INSTANCE;          public void test()        {            System.out.println("The Test!");        }    }</code></pre>    <p>测试代码：</p>    <pre>  <code class="language-java">package com.effective;      import java.lang.reflect.Constructor;    import java.lang.reflect.InvocationTargetException;      import com.effective.singleton.SingletonClass;      public class TestMain    {          public static void main(String[] args) throws NoSuchMethodException, SecurityException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException        {            Class<SingletonClass> classType = SingletonClass.class;            Constructor<SingletonClass> c = (Constructor<SingletonClass>) classType.getDeclaredConstructor();            c.setAccessible(true);            c.newInstance();        }    }</code></pre>    <p>运行结果：</p>    <pre>  <code class="language-java">Exception in thread "main" java.lang.NoSuchMethodException: com.effective.singleton.SingletonClass.<init>()        at java.lang.Class.getConstructor0(Unknown Source)        at java.lang.Class.getDeclaredConstructor(Unknown Source)        at com.effective.TestMain.main(TestMain.java:22)</code></pre>    <p>由此可见这种写法也可以防止单例模式被“攻击”。</p>    <p>而且这种写法也可以防止序列化破坏单例模式，具体不在举例了，有关序列化以及单例模式被序列化破坏可以参考博文《JAVA序列化》（链接：http://blog.csdn.net/u013256816/article/details/50474678）。</p>    <p>单元素的枚举类型已经成为实现Singleton模式的最佳方法。</p>    <p> </p>    <p>来自：http://www.importnew.com/22493.html</p>    <p> </p>