单例模式在多线程下
分隔
public class SingletonDemo {
private static SingletonDemo instance=null;
private SingletonDemo(){}
public static SingletonDemo getSingle() {
if (instance==null) {
instance=new SingletonDemo();
}
return instance;
}
} 分隔 public class SingletonDemo {
private static SingletonDemo instance=null;
private SingletonDemo(){
System.out.println("I am constructor");
}
public static SingletonDemo getSingle() {
if (instance==null) {
instance=new SingletonDemo();
}
return instance;
}
public static void main(String[] args) {
for (int i=0; i<10; i++) {
new Thread(() -> {SingletonDemo.getSingle();
},String.valueOf(i)).start();
}
}
} 
分隔
public class SingletonDemo {
private static SingletonDemo instance=null;
private SingletonDemo(){
System.out.println("I am constructor");
}
public static synchronized SingletonDemo getSingle() {
if (instance==null) {
instance=new SingletonDemo();
}
return instance;
}
public static void main(String[] args) {
for (int i=0; i<10; i++) {
new Thread(() -> {SingletonDemo.getSingle();
},String.valueOf(i)).start();
}
}
} 
分隔
DCL（Double Ckeck Lock）双端检锁机制
public static SingletonDemo getSingle() {
if (instance==null) {
synchronized(SingletonDemo.class) {
if (instance==null) {
instance=new SingletonDemo();
}
}
}
return instance;
} 分隔 
上面的代码是否一定正确呢？
不一定，指令重排
DCL（双端检索）机制不一定线程安全，
分隔
禁止指令重排
private static volatile SingletonDemo instance=null;
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