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单例模式：一个类只有一个实例方便控制并节约系统资源

优点：节省内存，加快访问速度，因此对象需要被公用的场合适合使用，如多个模块使用同一数据源连接对象等等

缺点：不适用于变化的对象，如果同一类型的对象总是在不同的用例场景发生变化，单例就会引起数据的错误，不能保存彼此的状态

饿汉模式的两种：

public class Singleton_hungry {	private final static Singleton_hungry singlen = new Singleton_hungry();	private Singleton_hungry() {}	public static Singleton_hungry  getInstance() {		return singlen;	}}class Singleton_hungry2{    private static Singleton_hungry2 instance;    static {        instance = new Singleton_hungry2();    }    private Singleton_hungry2() {}    public Singleton_hungry2 getInstance() {        return instance;    }}

懒汉的六种：

public class Singleton_lazy{	private static Singleton_lazy singleton= null;	private Singleton_lazy() {	}	public static Singleton_lazy getInstance() {		if(singleton==null) {			singleton = new Singleton_lazy();		}		return singleton;	}}class Singleton_lazy1{    private static Singleton_lazy1 singleton;    private Singleton_lazy1() {}    public static synchronized Singleton_lazy1 getInstance() {        if (singleton == null) {            singleton = new Singleton_lazy1();        }        return singleton;    }}class Singleton_lazy2{    private static Singleton_lazy2 singleton;    private Singleton_lazy2() {}    public static Singleton_lazy2 getInstance() {        if (singleton == null) {            synchronized (Singleton_lazy2.class) {                singleton = new Singleton_lazy2();            }        }        return singleton;    }}class Singleton_lazy3{	private static volatile Singleton_lazy3 singleton = null;	private Singleton_lazy3() {}	private static Singleton_lazy3 getInatance() {		if(singleton==null) {			synchronized(Singleton_lazy3.class) {				if(singleton==null) {					singleton = new Singleton_lazy3();				}			}		}		return singleton;	}}class Singleton_lazy4{	private Singleton_lazy4() {}	private static class SingletonInstance{		private static final Singleton_lazy4 singleton = new Singleton_lazy4();	}	public static Singleton_lazy4 getInstance() {		return SingletonInstance.singleton;	}}class Singleton_lazy5{	public enum singletonInstance{		singleton;		public void whatEverMethod() {		}	}}

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