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Singleton 单例模式
本系列为 https://github.com/iluwatar/java-design-patterns 阅读笔记
Eagerly
类一加载就实例化
/**
* Singleton class. Eagerly initialized static instance guarantees thread safety.
*/
public final class IvoryTower {
/**
* Private constructor so nobody can instantiate the class.
*/
private IvoryTower() {}
/**
* Static to class instance of the class.
*/
private static final IvoryTower INSTANCE = new IvoryTower();
/**
* To be called by user to obtain instance of the class.
*
* @return instance of the singleton.
*/
public static IvoryTower getInstance() {
return INSTANCE;
}
}
Lazy
保证线程安全有3中措施
- 对
getInstance()
方法添加synchronized
public final class ThreadSafeLazyLoadedIvoryTower {
private static ThreadSafeLazyLoadedIvoryTower instance;
private ThreadSafeLazyLoadedIvoryTower() {
// protect against instantiation via reflection
if (instance == null) {
instance = this;
} else {
throw new IllegalStateException("Already initialized.");
}
}
/**
* The instance gets created only when it is called for first time. Lazy-loading
*/
public static synchronized ThreadSafeLazyLoadedIvoryTower getInstance() {
if (instance == null) {
instance = new ThreadSafeLazyLoadedIvoryTower();
}
return instance;
}
- 双重检查锁 属性添加
volatile
修饰,instance
赋值时进行双重检查
public final class ThreadSafeDoubleCheckLocking {
private static volatile ThreadSafeDoubleCheckLocking instance;
/**
* private constructor to prevent client from instantiating.
*/
private ThreadSafeDoubleCheckLocking() {
// to prevent instantiating by Reflection call
if (instance != null) {
throw new IllegalStateException("Already initialized.");
}
}
/**
* Public accessor.
*
* @return an instance of the class.
*/
public static ThreadSafeDoubleCheckLocking getInstance() {
// local variable increases performance by 25 percent
// Joshua Bloch "Effective Java, Second Edition", p. 283-284
ThreadSafeDoubleCheckLocking result = instance;
// Check if singleton instance is initialized. If it is initialized then we can return the instance.
if (result == null) {
// It is not initialized but we cannot be sure because some other thread might have initialized it
// in the meanwhile. So to make sure we need to lock on an object to get mutual exclusion.
synchronized (ThreadSafeDoubleCheckLocking.class) {
// Again assign the instance to local variable to check if it was initialized by some other thread
// while current thread was blocked to enter the locked zone. If it was initialized then we can
// return the previously created instance just like the previous null check.
result = instance;
if (result == null) {
// The instance is still not initialized so we can safely (no other thread can enter this zone)
// create an instance and make it our singleton instance.
instance = result = new ThreadSafeDoubleCheckLocking();
}
}
}
return result;
}
}
- Singleton实例放到一个静态内部类中
public final class InitializingOnDemandHolderIdiom {
/**
* Private constructor.
*/
private InitializingOnDemandHolderIdiom() {}
/**
* @return Singleton instance
*/
public static InitializingOnDemandHolderIdiom getInstance() {
return HelperHolder.INSTANCE;
}
/**
* Provides the lazy-loaded Singleton instance.
*/
private static class HelperHolder {
private static final InitializingOnDemandHolderIdiom INSTANCE =
new InitializingOnDemandHolderIdiom();
}
}
enum为单例且线程安全
public enum EnumIvoryTower {
INSTANCE;
@Override
public String toString() {
return getDeclaringClass().getCanonicalName() + "@" + hashCode();
}
}