2.1-單例實現方式
摘要：
餓漢模式
執行緒安全，呼叫效率高，但是不能延時載入
public class SingletonObject1 {
private static final SingletonObject1 instance = new SingletonObject1();
...
餓漢模式
執行緒安全，呼叫效率高，但是不能延時載入
public class SingletonObject1 {
private static final SingletonObject1 instance = new SingletonObject1();
private SingletonObject1() {}
public static SingletonObject1 getInstance() {
return instance;
}
}
懶漢模式
執行緒不安全，會產生多個例項
public class SingletonObject2 {
private static SingletonObject2 instance;
private SingletonObject2() {}
public static SingletonObject2 getInstance() {
if (null == instance) {
instance = new SingletonObject2();
}
return SingletonObject2.instance;
}
}
getInstance()方法加鎖（效率低）
public class SingletonObject3 {
private static SingletonObject3 instance;
private SingletonObject3() {}
public synchronized static SingletonObject3 getInstance() {
if (null == instance) {
instance = new SingletonObject3();
}
return SingletonObject3.instance;
}
}
雙重檢測鎖模式
由於指令重排序問題，偶爾會出現異常，比如構造方法裡面有大量的操作，jvm可能不會等到所有的操作都執行完，才返回例項指標
public class SingletonObject4 {
private static SingletonObject4 instance;
private SingletonObject4() { }
public static SingletonObject4 getInstance() {
if (null == instance) {
synchronized (SingletonObject4.class) {
if (null == instance) {
instance = new SingletonObject4();
}
}
}
return SingletonObject4.instance;
}
}
5、雙重檢測鎖模式2
通過volatile解決【4】的問題，嚴格要求happen before，也就是在執行【讀】操作的時候必須看到完整的【寫】操作
public class SingletonObject5 {
private static volatile SingletonObject5 instance;
private SingletonObject5() {}
public static SingletonObject5 getInstance() {
if (null == instance) {
synchronized (SingletonObject4.class) {
if (null == instance) {
instance = new SingletonObject5();
}
}
}
return SingletonObject5.instance;
}
}
6、靜態內部類
懶載入、執行緒安全、效率高
public class SingletonObject6 {
private SingletonObject6() {}
private static class InstanceHolder {
private final static SingletonObject6 instance = new SingletonObject6();
}
public static SingletonObject6 getInstance() {
return InstanceHolder.instance;
}
}
7、列舉
列舉是執行緒安全的，而且只會被裝載一次
public class SingletonObject7 {
private SingletonObject7() {}
private enum Singleton {
INSTANCE;
private final SingletonObject7 instance;
Singleton() {
instance = new SingletonObject7();
}
public SingletonObject7 getInstance() {
return instance;
}
}
public static SingletonObject7 getInstance() {
return Singleton.INSTANCE.getInstance();
}