- 写一个线程安全的单例模式
- 1、饿汉式
- 2、懒汉式
- 3、使用双重校验 + volatile
- 4、枚举单例
- 5、Lock
- 6、AtomicReference + CAS
线程安全的单例模式有很多种写法：
1、饿汉式我们以jdk源码为例，Runtime类的源码：
类加载为静态变量赋值。
public class Runtime {
private static Runtime currentRuntime = new Runtime();
public static Runtime getRuntime() {
return currentRuntime;
}
private Runtime() {}
}
2、懒汉式
synchronized修饰静态方法。
public class Singleton {
private static Singleton instance;
private Singleton() {}
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
3、使用双重校验 + volatile
synchronized修饰代码块，volatile可见性。
public class Singleton {
private static volatile Singleton instance = null;
private Singleton() {}
public static Singleton getInstance() {
if (instance == null) { // 先判断实例是否存在，不存在再加锁
synchronized (Singleton.class) {
if (instance == null) { // 多线程情况下，只有一个能进入，避免创建多个对象。
return instance = new Singleton();
}
}
}
return instance;
}
}
4、枚举单例
一个枚举就是一个单例。
enum SingletonEnum {
INSTANCE;
}
5、Lock
public class Singleton {
private static Singleton instance;
private static final Lock lock = new ReentrantLock();
private Singleton() { }
public static Singleton getInstance() {
lock.lock();
try {
if (instance == null) {
instance = new Singleton();
}
return instance;
} finally {
lock.unlock();
}
}
}
6、AtomicReference + CAS
public class Singleton {
private static final AtomicReference instance = new AtomicReference();
private Singleton() { }
public static Singleton getInstance() {
for (;;) {
Singleton singleton = instance.get();
if (null != singleton) {
return singleton;
}
singleton = new Singleton();
if (instance.compareAndSet(null, singleton)) {
return singleton;
}
}
}
}
