单例模式的写法
//饿汉(线程不安全，不正确)
class Singleton1 {
private static Singleton1 instance = new Singleton1();
private Singleton1(){
}
public static Singleton1 getInstance(){
return instance;
}
}
//饱汉(线程安全但是高并发性能不高)
class Singleton2 {
private static Singleton2 instance;
private Singleton2(){
}
public static synchronized Singleton2 getSingleton2(){
if(instance == null){
instance = new Singleton2();
}
return instance;
}
}
//线程安全，性能又高，写法常见
class Singleton3{
private static Singleton3 instance;
private static byte[] lock = new byte[0];
private Singleton3(){
}
public static Singleton3 getInstance() {
if(instance == null){
synchronized (lock) {
if(instance == null){
instance = new Singleton3();
}
}
}
return instance;
}
}
//线程安全，性能又高的另一种，也很常见
class Singleton4 {
private static Singleton4 instance;
private static ReentrantLock lock = new ReentrantLock();
private Singleton4 () {
}
public static Singleton4 getInstance() {
if(instance == null){
lock.lock();
if(instance == null){
instance == new Singleton4();
}
lock.unlock();
}
return instance;
}
}
