Java的4种单例模式
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第一种：线程不安全，不正确的写法
class Singleton{
private static Singleton instance;
private Singleton(){
}
public static Singleton getInstance(){
if(instance==null){
instance=new Singleton();
}
return instance;
}
}
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第二种：线程安全，但高并发不是很理想的写法：
class Singleton{
private static Singleton instance;
private Singleton(){
}
public static synchronized Singleton getInstance(){
if(instance==null){
instance=new Singleton();
}
return instance;
}
}
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第三种：线程安全，性能又高，这种写法最为常见：
class Singleton{
private static Singleton instance;
private static byte[] lock=new byte[0];
private Singleton(){
}
public static synchronized Singleton getInstance(){
if(instance==null){
synchronized(lock){
if(instance==null){
instance=new Singleton();
}
}
}
return instance;
}
}
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第四种：线程安全，性能又高，这种写法也最为常见：
class Singleton{
private static Singleton instance;
private static ReentrantLock lock=new ReentrantLock ();
private Singleton(){
}
public static synchronized Singleton getInstance(){
if(instance==null){
lock.lock();
if(instance==null){
instance=new Singleton();
}
lock.unlock();
}
return instance;
}
}