Java單例模式實現的幾種方式，java例模式幾種

Java單例模式實現的幾種方式，java例模式幾種

單例模式好多書上都是這麼寫的：

public class SingleTon1 {private SingleTon1(){}private static SingleTon1 instance = null;public static SingleTon1 getInstance(){if(instance == null){instance = new SingleTon1();} return instance;}}

　　但是實際開發中是不會這麼寫的，因為有一個嚴重的問題，多線程並發訪問的時候，可能會產生多個執行個體！！

下面列舉幾個常用的方法：

1.使用synchronized 關鍵字

package singleton;public class SingleTon1 {private SingleTon1(){}private static SingleTon1 instance = null;//多線程問題解法一，但是效率不高！因為每次調用都會加鎖！public static synchronized SingleTon1 getInstance(){if(instance == null){instance = new SingleTon1();} return instance;}public void print(){System.out.println("thread_id:"+Thread.currentThread().getId());}private static Object object = new Object();//很巧妙的方法，只有在null的時候加鎖，之後就不加啦public static SingleTon1 getInstance2(){if(instance == null){synchronized (object){instance = new SingleTon1();    }}return instance;}}

　2.加鎖

package singleton;import java.util.concurrent.locks.ReentrantLock;public class SingleTon2 {    private SingleTon2(){}    private static ReentrantLock lock = new ReentrantLock();private static SingleTon2 instance = null;public void print(){System.out.println("thread_id:"+Thread.currentThread().getId());}    public static SingleTon2 getInstance2(){if(instance == null){lock.lock();if(instance == null){  //注意這裡還要判斷下！！instance = new SingleTon2();}    lock.unlock();}return instance;}}

　　3.利用靜態變數：

package singleton;public class SingleTon3 {    public static void print(){System.out.println("thread_id:"+Thread.currentThread().getId());}    public static Nested getNested(){return Nested.instance;}    //這個是單例建立的類    static class Nested{      private Nested(){    }    static Nested instance = new Nested();    }}

以上就是常用的建立單例的模式：

Test測試代碼：

package singleton;import singleton.SingleTon3.Nested;public class Test2 {public static void main(String[] args) {// TODO Auto-generated method stubNested singleton;Myrunnable mm = new Myrunnable();Myrunnable m1 = new Myrunnable();Myrunnable2 m2 = new Myrunnable2();new Thread(m1).start();new Thread(m2).start();if(m1.singleton == m2.singleton){  //是同一個System.out.println("是同一個");}else{System.out.println("不是同一個");} }}class Myrunnable implements Runnable{Nested singleton;@Overridepublic void run() {// TODO Auto-generated method stubsingleton = SingleTon3.getNested();SingleTon3.print();}} class Myrunnable2 implements Runnable{Nested singleton;@Overridepublic void run() {// TODO Auto-generated method stubsingleton = SingleTon3.getNested();SingleTon3.print();}    }

輸出：

是同一個
thread_id:11
thread_id:10