定義
??一個類至多有一個對象實(shí)例。這個實(shí)例被稱為單例。在使用時才初始化被稱為懶漢式單例，類一加載就初始化被叫做餓漢式單例。有8種常見實(shí)現(xiàn)方式。
實(shí)現(xiàn)
- 靜態(tài)常量法：可用
public class Singleton {
private final static Singleton instance = new Singleton();
private Singleton(){}
public static Singleton getInstance(){
return instance;
}
}
- 靜態(tài)代碼塊：可用
public class Singleton {
private static Singleton instance;
static { instance = new Singleton(); }
private Singleton() {}
public Singleton getInstance() {
return instance;
}
}
- 靜態(tài)方法：不可用
public class Singleton {
private static Singleton singleton;
private Singleton() {}
public static Singleton getInstance() {
if (singleton == null)
singleton = new Singleton();
return singleton;
}
}
??線程A進(jìn)入第7句但還未來得及初始化時，線程B在第5句判斷為null，所以也可以進(jìn)入第6句，有并發(fā)問題。
- 同步方法：可用但不推薦
public class Singleton {
private static Singleton singleton;
private Singleton() {}
public static synchronized Singleton getInstance() {
if (singleton == null)
singleton = new Singleton();
return singleton;
}
}
- 同步代碼塊：不可用
public class Singleton {
private static Singleton singleton;
private Singleton() {}
public static Singleton getInstance() {
if (singleton == null) {
synchronized (Singleton.class) {
singleton = new Singleton();
}
}
return singleton;
}
}
??線程A進(jìn)入第7句但還未來得及初始化時，線程B在第5句判斷為null，雖然得等到A初始化完成，但也可以進(jìn)入第7句，有并發(fā)問題。
- 雙重檢查：可用且推薦用
public class Singleton {
private static volatile Singleton singleton;
private Singleton() {}
public static Singleton getInstance() {
if (singleton == null) {
synchronized (Singleton.class) {
if (singleton == null) {
singleton = new Singleton();
}
}
}
return singleton;
}
}
- 靜態(tài)內(nèi)部類：可用且推薦用
public class Singleton {
private Singleton() {}
private static class SingletonInstance {
private static final Singleton INSTANCE = new Singleton();
}
public static Singleton getInstance() {
return SingletonInstance.INSTANCE;
}
}
- 枚舉：可用且推薦用
public enum Singleton {
INSTANCE;
public void whateverMethod() {
doSomethingHere...
}
}
