CS-Notes/notes/JVM.md

<!-- GFM-TOC -->
* [??????](#??????)
    * [1. ?????????](#1-?????????)
    * [2. Java ??????](#2-java-??????)
    * [3. ????????](#3-????????)
    * [4. Java ??](#4-java-??)
    * [5. ??????](#5-??????)
    * [6. ???????????](#6-???????????)
    * [7. ??????](#7-??????)
* [???????](#???????)
    * [1. ?<3F><>???????????????](#1-?<3F><>???????????????)
        * [1.1 ???<3F><>???](#11-???<3F><>???)
        * [1.2 ?????](#12-?????)
        * [1.3 ????????](#13-????????)
            * [1.3.1 ?????](#131-?????)
            * [1.3.2 ??????](#132-??????)
            * [1.3.3 ??????](#133-??????)
            * [1.3.4 ??????](#134-??????)
        * [1.3 ???????????](#13-???????????)
        * [1.4 finalize()](#14-finalize)
    * [2. ?????????](#2-?????????)
        * [2.1 ??? - ?????](#21-???---?????)
        * [2.2 ??????](#22-??????)
        * [2.3 ??? - ??????](#23-???---??????)
        * [2.4 ????????](#24-????????)
    * [3. ?????????](#3-?????????)
        * [3.1 Serial ?????](#31-serial-?????)
        * [3.2 ParNew ?????](#32-parnew-?????)
        * [3.3 Parallel Scavenge ?????](#33-parallel-scavenge-?????)
        * [3.4 Serial Old ?????](#34-serial-old-?????)
        * [3.5 Parallel Old ?????](#35-parallel-old-?????)
        * [3.6 CMS ?????](#36-cms-?????)
        * [3.7 G1 ?????](#37-g1-?????)
        * [3.8 ?????????????????](#38-?????????????????)
    * [4. ??????????????](#4-??????????????)
        * [4.1 ?????? Eden ????](#41-??????-eden-????)
        * [4.2 ????????????????](#42-????????????????)
        * [4.3 ??????????????????](#43-??????????????????)
        * [4.4 ????????????<3F><>?](#44-????????????<3F><>?)
        * [4.5 ????????](#45-????????)
    * [4.6 Full GC ?????????](#46-full-gc-?????????)
        * [4.6.1 ???? System.gc()](#461-????-systemgc)
        * [4.6.2 ??????????](#462-??????????)
        * [4.6.3 ???????????](#463-???????????)
        * [4.6.4 JDK 1.7 ?????????????????](#464-jdk-17-?????????????????)
        * [4.6.5 Concurrent Mode Failure](#465-concurrent-mode-failure)
* [????????](#????????)
    * [1 ???????????](#1-???????????)
    * [2. ?????????](#2-?????????)
    * [3. ????????](#3-????????)
        * [3.1 ????](#31-????)
        * [3.2 ???](#32-???)
        * [3.3 ???](#33-???)
        * [3.4 ????](#34-????)
        * [3.5 ?????](#35-?????)
    * [4. ???????](#4-???????)
        * [4.1 ???????????](#41-???????????)
        * [4.2 ???????????](#42-???????????)
        * [4.3 ?????????](#43-?????????)
* [JVM ????](#jvm-????)
    * [GC ???????](#gc-???????)
    * [GC ????????](#gc-????????)
<!-- GFM-TOC -->

# ??????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/dc695f48-4189-4fc7-b950-ed25f6c80f82.jpg)

????????????????<3F><>??????????????????

## 1. ?????????

?????????<3F><>????????????????????????????<3F><>??? Native ????????????

## 2. Java ??????

??? Java ????????<3F><>????????????????<3F><>???????????????????????????????????????????????????????????????????????????????? Java ?????????????????????

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

1. ???????????????????????????? StackOverflowError ????
2. ????<3F><>????????????????????<3F><>????? OutOfMemoryError ????

## 3. ????????

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

## 4. Java ??

???<3F><>???????????????????<3F><>

??????????????????????????????"GC ?? "???????????????????????<3F><>?????????Java ??????????????????????????????????????? Eden ???From Survivor ???To Survivor ???????

???????????<3F><>??????? -Xmx ?? -Xms ???????????????<3F><>?????????????????? OutOfMemoryError ????

## 5. ??????

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

?? Java ??????????????????<3F><>??????????????????????????????? OutOfMemoryError ????

????????????????????????????????????????????<3F><>??????????????????HotSpot ?????????????????????????????????

## 6. ???????????

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

??????Class ????<3F><>???????????????????????????????????????????????????????

???????????????<3F><>? String ??? intern() ?????????????????????

## 7. ??????

?? JDK 1.4 ????????? NIO ???????????????????Channel??????????Buffer???? I/O ?????????????? Native ?????????????????<3F><>??????????<3F><>?? Java ????? DirectByteBuffer ????????????????????<3F><>??????????????<3F><>????????????????????????????? Java ??? Native ????????????????

# ???????

?????????????????????????????????????????????<3F><>????????????????????????????????????????????????????????????????????????

???????????????? Java ???????????<3F><>?

## 1. ?<3F><>???????????????

### 1.1 ???<3F><>???

???????????????<3F><>????????????????????????????????? 1???????<3F><>????????? 1??

???<3F><>???? 0 ????????????

?????????????????????????????<3F><>??????????? 0?????? GC ?????????????

```java
objA.instance = objB;
objB.instance = objA;
```

### 1.2 ?????

??? GC Roots ???????????????????????????????????????????????????????

GC Roots ???????????????

1. ????????????????
2. ???????????????????????
3. ???????<3F><>????????????
4. ??????????????????

### 1.3 ????????

????????????<3F><>??????<3F><>?????????????????????????????????<3F><>?????????????????<3F><>???????????????<3F><>??<3F><>??

#### 1.3.1 ?????

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

```java
Object obj = new Object();
```

#### 1.3.2 ??????


?????????????????????<3F><>????

```java
Object obj = new Object();
SoftReference<Object> sf = new SoftReference<Object>(obj);
obj = null;
sf.get();
```

sf ??? obj ??????????????? sf.get() ??????????????????????????????????????????????????? null??

????????????????????????????????????????????????????????????????????????????????????????????<3F><>???????????????????????????????????????<3F><>?????


#### 1.3.3 ??????

??????<3F><>???????????????????????????????????????????????????????????????

```java
Object obj = new Object();
WeakReference<Object> wf = new WeakReference<Object>(obj);
obj = null;
wf.get();
wf.isEnQueued();
```

#### 1.3.4 ??????

?????????????????????????????????????????????????????????????<3F>q??????????????????????????????????????????????????<3F><>?????<3F><>????????????????????????????????????????

```java
Object obj = new Object();
PhantomReference<Object> pf = new PhantomReference<Object>(obj);
obj=null;
pf.get();
pf.isEnQueued();
```

### 1.3 ???????????

???????????????????????????<3F><>???

?????????????<3F><>???????????

???<3F><>????????????????????????????????????????????????<3F><>???

1. ???????<3F><>????????????????????? Java ???<3F><>??????????<3F><>??????
2. ???????? ClassLoader ??????????
3. ???????? java.lang.Class ??????????<3F>ʦ<EFBFBD>??????????????????<3F>ʦ<EFBFBD>???????????????????

??????? -Xnoclassgc ???????????????????<3F><>???

???????<3F><>??????????CGLib ?? ByteCode ??????????? JSP ??? OSGo ???????????? ClassLoader ????????????????????<3F><>?????????????????????????

### 1.4 finalize()

???????????????????????????<3F><>????? finalize() ????????????<3F><>???????????<3F><>?????????????<3F><>????????????????

finalize() ???? C++ ???<3F>y??????????????????????????????? try-finally ????????????????????<3F><>??????<3F><>?????????????????????????????????????????<3F><>????<3F><>?

## 2. ?????????

### 2.1 ??? - ?????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/a4248c4b-6c1d-4fb8-a557-86da92d3a294.jpg)

???????????????<3F><>???????????

????

1. ???????????<3F><>???????
2. ????????????

???????????????????<3F><>????

### 2.2 ??????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/e6b733ad-606d-4028-b3e8-83c3a73a3797.jpg)

????<3F><>?????<3F><>???????<3F><>?????????????<3F><>??????????????????????????????????????<3F><>????????<3F><>??????????????????

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

??????????????????????????????????????????????????????<3F><>?????<3F><>???????<3F><>???????????? Eden ?????????<3F><>?? Survior ????????? Eden ??????????? Survivor???????????? Eden ?? Survivor ?<3F><>???????????????????????? Survivor ????????????? Eden ?? Survivor??HotSpot ??????? Eden ?? Survivor ???<3F><>???????? 8:1?????????????????? 90 %???????<3F><>????<3F><>??? 10% ????????????? Survivor ?????????????????????????????<3F><>????????????????????????

### 2.3 ??? - ??????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/902b83ab-8054-4bd2-898f-9a4a0fe52830.jpg)

?????<3F><>??????????????????????????????????????<3F><>

### 2.4 ????????

????????????????<3F><>?????????????????????????????????????????????????<3F><>??????<3F><>????????????????????

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

1. ????????????????
2. ???????????? - ???? ???? ??? - ???? ????

## 3. ?????????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/c625baa0-dde6-449e-93df-c3a67f2f430f.jpg)

?????? HotSpot ??????<3F><>? 7 ?????????????????????????????????????<3F><>?

### 3.1 Serial ?????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/22fda4ae-4dd5-489d-ab10-9ebfdad22ae0.jpg)

??????????????????????<3F><>????????????????????????????????????????????????????????????????????????????????????????????????

???????????<3F><>????????? CPU ??????????????????????????????????????????????<3F><>???

?? Client ??<3F><>????<3F><>?????????????????????????????????????????????????????????????????????????????????????????????????????????????????????

### 3.2 ParNew ?????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/81538cd5-1bcf-4e31-86e5-e198df1e013b.jpg)

???? Serial ???????????<3F><>??

?? Server ??????????????????????????????????????????????????? Serial ???????????????? CMS ?????????????

???????????????? CPU ???????????????? -XX:ParallelGCThreads ?????????????????

### 3.3 Parallel Scavenge ?????

????<3F><>????????????

?????????????????????????????????????????????????????????????????????????????????????????????????????????????????? CPU ??????????????????????????????

??????????????????????????????????????????????????????<3F><>????????????????<3F><>??????? CPU ?????????????????????????????????????????????????????

?????????????????????????????????????????????????????? -XX:MaxGCPauseMillis ??????????????????????<3F><>?? -XX:GCTimeRatio ???????????? 0 ??<3F><>?? 100 ???????????????????????????????????????????????????????????????<3F><>????????????????????????????????

????????????? -XX:+UseAdaptiveSizePolicy?????????????????????????????????????????????<3F><>??-Xmn????Eden ?? Survivor ?????????-XX:SurvivorRatio?????????????????????-XX:PretenureSizeThreshold???????????????????????????????????????????????????????????<3F><>???????????????????????????????????????????? GC ??????????????GC Ergonomics??????????????????????? ParNew ?????????????????

### 3.4 Serial Old ?????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/08f32fd3-f736-4a67-81ca-295b2a7972f2.jpg)

Serial Old ?? Serial ?????????????<3F><>?????? Client ????????????<3F><>???????? Server ??????????????????

1. ?? JDK 1.5 ??????<3F><>??Parallel Old ????????????? Parallel Scavenge ???????????<3F><>?
2. ??? CMS ????????????????????????? Concurrent Mode Failure ???<3F><>?

### 3.5 Parallel Old ?????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/278fe431-af88-4a95-a895-9c3b80117de3.jpg)

?? Parallel Scavenge ?????????????<3F><>??

?????????????? CPU ??????<3F><>?????????????????? Parallel Scavenge ?? Parallel Old ???????

### 3.6 CMS ?????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/62e77997-6957-4b68-8d12-bfd609bb2c68.jpg)

CMS??Concurrent Mark Sweep?????? Mark Sweep ?????????????? ??? - ??? ???????

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

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

1. ??????????????????? GC Roots ???????????????????????????
2. ???????????? GC Roots Tracing ??????????????????????<3F><>???????????????
3. ???<3F><>?????????????????????????????????????????<3F><>?????????????????????????????????
4. ??????????????????

???????????<3F><>????????????????????????<3F><>??????????????????????????????????????????

???????????

1. ?? CPU ??????<3F><>?CMS ??????????????????? (CPU ???? + 3) / 4???? CPU ???? 4 ?????CMS ??????????????????<3F><>????????? CPU ???????????????????????????????????????????????????????????????????? 50%?????????????????????????????????????????????????????? CPU ?????????

2. ??????????????????????????????????????????????????????????????????????????????????????????????????????????CMS ????????????<3F><>????????????????????? GC ?????????????????????????????????????????????????????????????????????????<3F><>??????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????<3F><>???????? -XX:CMSInitiatingOccupancyFraction ???????????????????????????????JDK 1.5 ???????????? 68?????????????????? 68% ?????????????????????????????????????????????????????<3F><>????????? Concurrent Mode Failure?????????????????????????????? Serial Old ??????????????????????????????

3. ??? - ??????????????????????????????????<3F><>?????????????????????????????????????????????????????<3F><>??????????? Full GC??

### 3.7 G1 ?????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/f99ee771-c56f-47fb-9148-c0036695b5fe.jpg)

G1??Garbage-First??????????????????????????????????????????????????????????????????HotSpot ?????????????????????????????<3F><>???????<3F>I?? JDK 1.5 ?<3F><>????? CMS ???????

??????????

- ??????????????????? CPU ???????????????????? CPU ????????????
- ??????????????????????????????????????????????????????????????? GC ?????????????<3F><>?????????????????????????????????????? GC ???????????????????<3F><>????
- ????????????????????????? - ??????????????????????????? Region ???????????????????????????????<3F><>????????????????????????
- ??????????????????? CMS ?????????????????????? G1 ?? CMS ??????????? G1 ???????????????????????????????????????????????????????????? M ???????????????????? GC ???????<3F><>??? N ??????????????? Java??RTSJ??????????????????????

?? G1 ???????????????????????<3F><>???????????????????????????? G1 ????????????Java ?????<3F><>??????????????<3F><>??????????? Java ???????????<3F><>???????????Region?????????????????????????????????????????????????????????????????????????? Region???????????????????

??????????????????????????????????????<3F><>????????????? Java ???<3F><>??????????????????????????? Region ??????????????????<3F><>???????????????<3F><>??????????????????????????????????????<3F><>???<3F><>??????????????????????????? Region????????? Garbage-First ??????????????????? Region ???????????????????????????????????????????????????????????????????<3F><>???

Region ????????????????????????????? Region ?<3F><>??????????? Java ???????????????<3F><>?????????????????????????????????????????? Java ???????????????????? GC <20><>???????????????????????????????? Region ?????????????????? Remembered Set????????????????? Reference ????????????<3F><>??????????????? Write Barrier ????<3F><>?<3F><>????????? Reference ????????????????? Region ??<3F><>???????????? CardTable ???????????????????????????????? Region ?? Remembered Set ??<3F><>?????????????????? GC ??????????<3F><>?<3F><>??? Remembered Set ????????????????????????????

???????????? Remembered Set ???????G1 ??????????????????????????????<3F>s

1. ??????
2. ???????
3. ??????????????????????????????????????????????<3F><>??????????????????????????????????????<3F><>????????? Remembered Set Logs ???<3F><>????????????? Remembered Set Logs ?????????? Remembered Set ?<3F><>?????????????????????????<3F><>?
4. ??????????????? Region ?<3F><>???????????????????????????????? GC ????????????????????????????????????????????????????<3F><>?????????????????? Region??????????????????????????????????????????<3F><>???

### 3.8 ?????????????????

| ????? | ???<3F><>????? or ???? | ?????? / ????? | ?? | ??? | ???<3F><>??? |
| --- | --- | --- | --- | --- | --- |
| **Serial** | ???? | ?????? | ?????? | ?????????? | ?? CPU ??????? Client ?? |
| **Serial Old** | ???? | ????? | ??? - ???? | ?????????? | ?? CPU ??????? Client ????CMS ?????? |
| **ParNew** | ???? | ?????? | ?????? | ?????????? | ?? CPU ??????? Server ?????? CMS ??? |
| **Parallel Scavenge** | ???? | ?????? | ?????? | ?????????? | ????????????????????????? |
| **Parallel Old** | ???? | ????? | ??? - ???? | ?????????? | ????????????????????????? |
| **CMS** | ???? | ????? | ??? - ??? | ?????????? | ?????????????? B/S ?????????? Java ??? |
| **G1** | ???? | both | ??? - ???? + ?????? | ?????????? | ?????????????????<3F>I CMS |

## 4. ??????????????

### 4.1 ?????? Eden ????

??????????????????????? Eden ???????? Eden ?????????????? Minor GC??

### 4.2 ????????????????

?? -XX:PretenureSizeThreshold ????????????????????????????????????? Eden ???? Survivor ???????????<3F><>???
### 4.3 ??????????????????

JVM ?????????????????????? Minor GC ????????? Survivor ????????????? Survivor ????????? 1?????????? Minor GC ??????????????? 1???????????????????????????????? 15 ????? -XX:MaxTenuringThreshold ???????


### 4.4 ????????????<3F><>?

?? Survivor ????????????<3F><>????<3F><>?????? Survivor ???????????????????????????????????????????

### 4.5 ????????

????? Minor GC ????JVM ?????????????????????????????????????<3F><>?????????????? Minor GC ???????????????????????????????????????????????<3F><>????????????????????<3F><>???????????? Minor GC??<3F><>???????? Full GC??

## 4.6 Full GC ?????????

???? Minor GC?????????????????? Eden ???????????????????? Minor GC???? Full GC ?????????????????????

### 4.6.1 ???? System.gc()

??????????????? JVM ???? Full GC??????????????????????????????????? Full GC????????? Full GC ????????????????<3F><>???????????????????????????????????????????????????????????<3F><>????? -XX:+ DisableExplicitGC ????? RMI ???? System.gc()??

### 4.6.2 ??????????

??????????????????????????????????????????????????????????????????????? Full GC ??????????????????????? Java.lang.OutOfMemoryError: Java heap space ????????????????????? Full GC??????????????????????? Minor GC ??<3F><>????????????????????????????<3F><>??????????????????<3F><>

### 4.6.3 ???????????

?????????? Minor GC ???????????????????????????????? HandlePromotionFailure ????????????? Full GC??

### 4.6.4 JDK 1.7 ?????????????????

?? JDK 1.7 ???????HotSpot ??????<3F><>??????????????????????????<3F><>?????<3F><> class ??????????????????????????????????????????????????????????????Permanet Generation ????????????<3F><>????????? CMS GC ????????????? Full GC????????? Full GC ?????????????? JVM ?????????????????java.lang.OutOfMemoryError: PermGen space ????? PermGen ?????? Full GC ????????????????? PermGen ????????? CMS GC??

?? JDK 1.8 ?????????<3F>I?????????????????????????????????<3F><>??????????? Full GC ????????????

### 4.6.5 Concurrent Mode Failure

??? CMS GC ??????????<3F><>????????????????????????????????????????? CMS GC ??????????????????????????????? Full GC??????? Concurrent Mode Failure ????????? Full GC??

# ????????

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

## 1 ???????????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/32b8374a-e822-4720-af0b-c0f485095ea2.jpg)

???????? 7 ????<3F><>?

- **?????Loading??**
- **?????Verification??**
- **?????Preparation??**
- **??????Resolution??**
- **???????Initialization??**
- ????Using??
- <20><>???Unloading??

???<3F><>??????????<3F><>???????????????????????????????????? Java ???????

## 2. ?????????

??????<3F>Z?<3F><>????????????????<3F><>????????<3F>Z???<3F><>?????????????????????????????<3F><>??????( ???????????????????????? )

1. ???? new??getstatic??putstatic??invokestatic ????????????????????????<3F><>??<3F><>?????????????????????????????????????? 4 ??????????????? new ????????????????????????????????????<3F><>??? final ???<3F><>???????????????????????????<3F><>????????????????????????????????

2. ??? java.lang.reflect ?????????????<3F><>?????????????????<3F><>??<3F><>????????????????????????

3. ????????????????????????<3F><>????<3F><>??<3F><>???????????????????<3F><>?????????

4. ?????????????????????????????<3F><>????????? main() ?????????????????????????????????

5. ????? jdk1.7 ???????????????????? java.lang.invoke.MethodHandle ?????????????? REF_getStatic, REF_putStatic, REF_invokeStatic ???????????????????????????????????<3F><>??<3F><>??????????????????????????

???? 5 ??????<3F><>????????????????????????<3F><>??????????????????????????????????????????????<3F><>??????????????????????

1\. ???????????????????<3F><>?????????????????

```java
System.out.println(SubClass.value); // value ????? SuperClass ?<3F><>???
```

2\. ??????<3F>E????????????????????????????<3F><>?????????????<3F><>???????????????????????????????????????? Object ?????????<3F><>????????????????????

```java
SuperClass[] sca = new SuperClass[10];
```

3\. ??????????<3F><>????????????????<3F><>??????????????????????<3F>i?????????????????<3F>i?????????????

```java
System.out.println(ConstClass.HELLOWORLD);
```

## 3. ????????

???????????????????????????????? 5 ????<3F><>?

### 3.1 ????

?????????????????<3F><>???????????

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

1. ????????????????????????????????????????
2. ????????????????????<3F><>????????????????????<3F><>????
3. ???????????????????????? Class ??????????????????????????????????

???<3F><>??????????????????<3F><>???<3F><>????

- ?? ZIP ???????????????????????? JAR??EAR??WAR ??????????
- ???????<3F><>?????????????????????? Applet??
- ???????????????????????????????????????????? java.lang.reflect.Proxy ?<3F><>????????? ProxyGenerator.generateProxyClass ????????????????????
- ??????????????????????? JSP ???????? JSP ??????????? Class ??
- ????????????????????????????????<3F><>?<3F><>???????????? SAP Netweaver?????????????????????????????????????????????
...

### 3.2 ???

??? Class ???????????<3F><>??????????????????????????????<3F><>????????????????

????????? 4 ????<3F><>?

1. ?????????
2. ???????????????????????????????????????
3. ??????????????????????????????????????????????????????????????????????????<3F><>???????
4. ???????????

### 3.3 ???

???????? static ???<3F><>????????????????????????<3F>g???<3F><>????????????????????<3F><>

????????????????<3F><>?????<3F><>???????????????????????????????? Java ???<3F><>?

???????? 0 ????????????????? value ???????? 0 ?????? 123??

```java
public static int value = 123;
```

??????????????????????????????<3F><>??????????????? 0??

```java
public static final int value = 123;
```

### 3.4 ????

?????????????????<3F>I?????????????

### 3.5 ?????

???????<3F><>??????????????? &lt;clinit>() ??????????

???????<3F><>?????????????????????????????????????<3F><>?????????????????????????????????????????????????

&lt;clinit>() ???????????????

- ???????????????????????????????????????????<3F><>static{} ?<3F><>?<3F><>???????????????????????????????????????<3F><>??????????????????????????????????????????????????????????????????????????????????????????????????

```java
public class Test {
    static {
        i = 0;                // ????????????????????????
        System.out.print(i);  // ????????????????????????<3F><>?
    }
    static int i = 1;
}
```

- ?????????????????????????? &lt;init>()??????????????????????????????????????????????????? &lt;clinit>() ????????????????? &lt;clinit>() ?????????<3F><>??????????????<3F><>??????? &lt;clinit>() ??????????? java.lang.Object??

- ???????? &lt;clinit>() ????????<3F><>??????<3F><>??????<3F><>???????????????????????????????????????????

```java
static class Parent {
        public static int A = 1;
        static {
            A = 2;
        }
}

static class Sub extends Parent {
        public static int B = A;
}

public static void main(String[] args) {
        System.out.println(Sub.B);  // ????????????<3F><>???????? A??????? 2
}
```

- &lt;clinit>() ??????????????????????????????<3F><>???????????<3F><>???<3F><>???????????????????????????????????? &lt;clinit>() ??????

- ????<3F><>?????????????<3F><>??????????????????????????????????????????????? &lt;clinit>() ???????????????????????<3F><>??? &lt;clinit>() ?????????????<3F><>????? &lt;clinit>() ????????<3F><>???????<3F><>????????????????????????????????????????????????????????<3F><>??? &lt;clinit>() ??????

- ?????????????? &lt;clinit>() ??????????????<3F><>????????????????????????????????????????????????????????? &lt;clinit>() ???????????????????????????????????? &lt;clinit>() ??????????????????? &lt;clinit>() ???????<3F><>??????????????????????????????????????<3F><>????????????<3F><>?

## 4. ???????

????????????????????<3F><>????????????????????????????????????????? ( ??????? )???????????? Java ???????????????????<3F><>???????????????????????????????????????????????????????????

### 4.1 ???????????

???????????????????????????????????????????????????? Java ??????<3F><>?<3F><>??????????????????????????????????????????????????????????????????????????????????????? Class ????? equals() ??????isAssignableFrom() ??????isInstance() ????????????????????? instanceof() ????????????????????<3F><>?????????????????????????????????????????????????????<3F><>??????????????????????? Class ???????????????????????????????????????????????????????????????

### 4.2 ???????????

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

??????????????????Bootstrap ClassLoader???????????????? C++ ??????????????????????????????????????????????????<3F><>???? Java ???????????????????????????????????? java.lang.ClassLoader??

?? Java ??????????????????????????????????<3F><>??

- ?????????????Bootstrap ClassLoader?? ????????????????? <JAVA_HOME>\lib ???<3F><>??????? -Xbootclasspath ???????????<3F><>???<3F><>?????????????????????????????????? rt.jar???????????????????? lib ??????????????????????????????<3F><>? ???????????????? Java ??????????????????<3F><>????????????????????????????????????????????????????? null ???<3F>r?<3F><>?

- ????????????Extension ClassLoader?? ?????????????? ExtClassLoader??sun.misc.Launcher$ExtClassLoader????????????? <Java_Home>/lib/ext ????? java.ext.dir ???????????<3F><>???<3F><>????????????????<3F><>????????????????????????????

- ??<3F><>????????????Application ClassLoader?? ?????????????? AppClassLoader??sun.misc.Launcher$AppClassLoader??????????????????????? ClassLoader ?<3F><>? getSystemClassLoader() ?????????????????????????????????????????????<3F><>????ClassPath??????????????????????????????????????????????<3F><>?????????????????????????????????????????????????????????????

### 4.3 ?????????

??<3F><>?????????????????????????<3F><>?????????<3F><>?????????????????????????????????????????????????<3F><>??????????????????????????Parents Delegation Model??????????????????????????????????????????????????????????????????????????????????????????????Composition??????????????????????<3F><>?Inheritance??????????

![](https://github.com/CyC2018/InterviewNotes/blob/master/pics/2cdc3ce2-fa82-4c22-baaa-000c07d10473.jpg)

**????????**

??????????????????????????????????????????????????????????????????????????????????<3F><>?????????????????<3F><>?<3F><>??????<3F><>?????????????????????????????????????<3F><>???<3F><>??????????????????????????????????????<3F><>?????????????????????????????????????

**???**

??????????????????????????????????? Java ??????????????????????????????????????<3F><>?????????? java.lang.Object????????? rt.jar ?<3F><>??????????????????????????????????????????????????????????????<3F><>??????? Object ???????????????????????<3F><>????????????????????????????????????????????<3F><>???????????????<3F><>??????????java.lang.Object ?????????????? ClassPath ?<3F><>??????<3F><>????????????? Object ????????????????????????????<3F><>????? rt.jar ????????????????? Java ??????????????????????????????????????<3F><>?

**???**

```java
protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException{
    //check the class has been loaded or not
    Class c = findLoadedClass(name);
    if(c == null) {
        try{
            if(parent != null) {
                c = parent.loadClass(name, false);
            } else{
                c = findBootstrapClassOrNull(name);
            }
        } catch(ClassNotFoundException e) {
            //if throws the exception , the father can not complete the load
        }
        if(c == null) {
            c = findClass(name);
        }
    }
    if(resolve) {
        resolveClass(c);
    }
    return c;
}
```

# JVM ????

## GC ???????

| ???? | ???? |
| --- | --- |
| -Xms | ???????????<3F><> |
| -Xmx | ????????? |
| -Xmn | ????????<3F><> |
| -XX:PermSize | ????????????<3F><> |
| -XX:MaxPermSize | ???????????? |

## GC ????????

| ???? | ???? |
| --- | --- |
| -XX:+UseSerialGC | ?????????????? |
| -XX:+UseParallelGC | ?????????????? |
| -XX:+UseConcMarkSweepGC | ???????????????????? |
| -XX:ParallelCMSThreads= | ???????????????????? = ???????????? |
| -XX:+UseG1GC | G1 ?????????? |

```java
java -Xmx12m -Xms3m -Xmn1m -XX:PermSize=20m -XX:MaxPermSize=20m -XX:+UseSerialGC -jar java-application.jar
```