单例模式好多书上都是这么写的：
public class SingleTon1 {
private SingleTon1(){
}
private static SingleTon1 instance = null;
public static SingleTon1 getInstance(){
if(instance == null){
instance = new SingleTon1();
}
return instance;
}
}
但是实际开发中是不会这么写的，因为有一个严重的问题，多线程并发访问的时候，可能会产生多个实例！！
下面列举几个常用的方法：
1.使用synchronized 关键字
package singleton;
public class SingleTon1 {
private SingleTon1(){
}
private static SingleTon1 instance = null;
//多线程问题解法一，但是效率不高！因为每次调用都会加锁！
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的时候加锁，之后就不加啦
public static SingleTon1 getInstance2(){
if(instance == null){
synchronized (object){
instance = new SingleTon1();
}
}
return instance;
}
}
2.加锁
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){ //注意这里还要判断下！！
instance = new SingleTon2();
}
lock.unlock();
}
return instance;
}
}
3.利用静态变量：
package singleton;
public class SingleTon3 {
public static void print(){
System.out.println("thread_id:"+Thread.currentThread().getId());
}
public static Nested getNested(){
return Nested.instance;
}
//这个是单例创建的类
static class Nested{
private Nested(){
}
static Nested instance = new Nested();
}
}
以上就是常用的创建单例的模式：
Test测试代码：
package singleton;
import singleton.SingleTon3.Nested;
public class Test2 {
public static void main(String[] args) {
// TODO Auto-generated method stub
Nested 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){ //是同一个
System.out.println("是同一个");
}else{
System.out.println("不是同一个");
}
}
}
class Myrunnable implements Runnable{
Nested singleton;
@Override
public void run() {
// TODO Auto-generated method stub
singleton = SingleTon3.getNested();
SingleTon3.print();
}
}
class Myrunnable2 implements Runnable{
Nested singleton;
@Override
public void run() {
// TODO Auto-generated method stub
singleton = SingleTon3.getNested();
SingleTon3.print();
}
}
输出：
是同一个
thread_id:11
thread_id:10