单例模式

饿汉模式

public class EagerSingleton {
    private static final EagerSingleton instance = new EagerSingleton();
    private EagerSingleton() {
    }
    public static EagerSingleton getInstance() {
        return instance;
    }
}

懒汉模式

public class LazySingleton {
    private static volatile LazySingleton instance = null;
    private LazySingleton() {
    }
    public static LazySingleton getPerson() {
        if (instance == null) {
            synchronized (LazySingleton.class) {
                if (instance == null) {
                    instance = new LazySingleton(); 
                }
            }
        }
        return instance;
    }
}

如果不加volatile指令重排的情况下，单例会变成多例。
http://ifeve.com/jmm-faq-dcl/

看似简单的一段赋值语句：instance = new LazySingleton();，其实JVM内部已经转换为多条指令： memory = allocate(); //1：分配对象的内存空间 ctorInstance(memory); //2：初始化对象 instance = memory; //3：设置instance指向刚分配的内存地址 但是经过重排序后如下： memory = allocate(); //1：分配对象的内存空间 instance = memory; //3：设置instance指向刚分配的内存地址，此时对象还没被初始化 ctorInstance(memory); //2：初始化对象 可以看到指令重排之后，instance指向分配好的内存放在了前面，而这段内存的初始化被排在了后面，在线程A初始化完成这段内存之前，线程B虽然进不去同步代码块，但是在同步代码块之前的判断就会发现instance不为空，此时线程B获得instance对象进行使用就可能发生错误。

内部类模式

public class IoDHSingleton {
    private IoDHSingleton() {
    }
    public static IoDHSingleton getInstance() {
        return HolderClass.instance;
    }
    private static class HolderClass {
        private final static IoDHSingleton instance = new IoDHSingleton();
    }
}

如何破坏单例

• 反射
• 序列化

JDK中的单例

public class Runtime {
    private static Runtime currentRuntime = new Runtime();
    /**
     * Returns the runtime object associated with the current Java application.
     * Most of the methods of class <code>Runtime</code> are instance
     * methods and must be invoked with respect to the current runtime object.
     *
     * @return  the <code>Runtime</code> object associated with the current
     *          Java application.
     */
    public static Runtime getRuntime() {
        return currentRuntime;
    }
    /** Don't let anyone else instantiate this class */
    private Runtime() {}
}