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1.???????

??????????????????????????????????????????????????????????????????(????)?

2.?????????

????????????????????????????????(???????)??????????????????????????????(??????session???)?

3.???????????

• ????????
• ??????????
• ?????????
• ?????

4.???????????

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?????

????????????????????????????????????????????????????????????

1.???????????

public class SingleTon01 {    //1. ??????, ???new    private SingleTon01() {}    //2.??????????    private static final SingleTon01 instance = new SingleTon01();    //3.??????????????????    public static SingleTon01 getInstance() {        return instance;    }}

??????????????????????????????????????????????????????????????????????????????????????????????

???????????????????????????????? ??????????????????????????????????????????

2.???????

public class SingleTon02 {    //1. ??????, ???new    private SingleTon02() {}    //2.??????????    private static SingleTon02 instance;    static {        instance = new SingleTon02();    }    //3.??????????????????    public static SingleTon02 getInstance() {        return instance;    }}

?????

??????????????????????????????????????????????????????

1.??????????

public class SingleTon03 {    //1. ??????, ???new    private SingleTon03() {}    private static SingleTon03 singleTon03;    public static SingleTon03 getInstance() {        if (singleTon03 == null) {            singleTon03 = new SingleTon03();        }        return singleTon03;    }}

????????????getInstance()??????????????singleTon03?????????????????????????????????????????????????????????????????A??????if??????????????????B???????B????????????????singleTon03????????????A?????A????????????????????????

????????????

2.?synchronized?????????

public class SingleTon04 {    //1. ??????, ???new    private SingleTon04() {}    private static SingleTon04 singleTon03;    public static synchronized SingleTon04 getInstance() {        if (singleTon03 == null) {            singleTon03 = new SingleTon04();        }        return singleTon03;    }}

????A?B????????getInstance?????????synchronized??????????????????????????????????if????????????????????????synchronized????????????????????

???????????????????????????????????????getInstance()?????? ??????????????????????????????????? ??return???????????????

3.???????Double-Check

public class SingleTon05 {    //1. ??????, ???new    private SingleTon05() {}    private static SingleTon05 singleTon03;    public static SingleTon05 getInstance() {        if (singleTon03 == null) {            synchronized (SingleTon05.class) {                singleTon03 = new SingleTon05();            }        }        return singleTon03;    }}

??????????????????????????A???B????getInstance??????A???B??????????if????????????synchronized?????????????

4.?????Double-Check??

public class SingleTon06 {    //1. ??????, ????new    private SingleTon06() {}    private static volatile SingleTon06 singleTon06;    public static SingleTon06 getInstance() {        if (singleTon06 == null) {            synchronized (SingleTon06.class) {                if (singleTon06 == null) {                    singleTon06 = new SingleTon06();                }            }        }        return singleTon06;    }}

?????????????????????????????????????????????????Double-Check??????

5.???????

public class SingleTon07 {    //1. ??????, ???new    private SingleTon07() {}    private static class singleTon07Instance {        private static final SingleTon07 INSTANCE = new SingleTon07();    }    public static SingleTon07 getInstance() {        return singleTon07Instance.INSTANCE;    }}

???????????????????Java?????????????????????????????????????????????????????????????????????????????

??????

public enum SingleTon08 {    INSTANCE;    public void method() {}}

??jdk1.5????????????????????????

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