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Java中单利模式有三种： 懒汉式、饿汉式、登记试

饿汉式：

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

懒汉式：

public class Singleton {    private Singleton() {}    private static Singleton instance = null;    public synchronized static Singleton getInstance() {        if(instance == null) {            instance = getInstance();        }        return instance;    }}

登记式：

import java.util.HashMap;import java.util.Map;public class Singleton {    private static Map
     
       map = new HashMap
      
       ();    static {        Singleton single = new Singleton();        map.put(single.getClass().getName(), single);    }    protected Singleton() {}    public static Singleton getInstance(String name) {        if(name == null) {            name = Singleton.class.getName();        }        if(map.get(name) == null) {            try {                map.put(name, (Singleton)Class.forName(name).newInstance());            } catch(Exception e) {                e.printStackTrace();            }        }        return map.get(name);    }}