本文将通过最基本的垃圾收集器（Serial + Serial Old），简单地讲述JVM内存分配和回收过程中的3个基本的晋升规则：大对象直接晋升、对象年龄晋升、动态晋升
代码托管在：https://github.com/zhongmingmao/jvm_demo

-XX:+UseSerialGC

新生代采用Serial垃圾收集器（Copying算法，单线程，Stop The World）
老年代采用Serial Old垃圾收集器（Mark-Compact算法，单线程，Stop The World）

Minor GC / Major GC / Full GC

常规理解

Eden空间不够 ➔ Minor GC ➔ 回收Young Generation
Old Generation空间不够 ➔ Major GC ➔ 回收Old Generation
Method Area（Java 8开始由MetaSpace实现，之前由Permanent Generation实现）空间不够 ➔ Full GC ➔ 回收Young Generation+Old Generation+Method Area

最难区分的是Major GC和Full GC，其实并没有明确规定两者的区别，因此不要以Minor GC、Major GC、Full GC的方式来思考问题，而应该关注

GC是否需要Stop-The-World
GC能否并发（不是并行，是并发！）
应用的延迟和吞吐量

本文认为Major GC ≈ Full GC，不作区分

Minor GC

发生在新生代，当Eden区域没有足够空间进行分配
Java对象大多具有短命的特性
Minor GC非常频繁，速度也比较快
Minor GC会造成Stop-The-World，但由于新生代的对象为大多为短命对象，因此由Stop-The-World而造成的延迟可以忽略

Major GC / Full GC

发生在老年代
出现Major GC，经常伴随至少一次Minor GC
速度比较慢，SpeedOf(Minor GC) ≈ 10 * SpeedOf(Major GC)
Major GC也会造成Stop-The-World，但由于老年代的对象为大多为长命对象，因此由Stop-The-World而造成的延迟可能会比较大，因此才会出现并发收集器：CMS和G1（Java 9默认）

大对象直接晋升

代码

// JVM Args : -Xms20m -Xmx20m -Xmn10m -XX:SurvivorRatio=8 -XX:+UseSerialGC -XX:+PrintGCDetails
// 堆大小：20M
// 新生代大小：10M
// Eden区大小：8M
// Survivor区大小：1M
// 老年代大小：10M
public class MinorGCAndFullGC {
    private static final int _1MB = 1024 * 1024;

    public static void main(String[] args) {
        System.gc();
        byte[] b1 = new byte[5 * _1MB];
        byte[] b2 = new byte[5 * _1MB];// 1次Minor GC
        byte[] b3 = new byte[5 * _1MB];// 1次Minor GC + 1次Full GC ⇒ OOM
    }
}

运行结果

[Full GC (System.gc()) [Tenured: 0K->467K(10240K), 0.0037172 secs] 2217K->467K(19456K), [Metaspace: 3177K->3177K(1056768K)], 0.0037645 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [DefNew: 5447K->4K(9216K), 0.0036997 secs] 5915K->5592K(19456K), 0.0037186 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [DefNew: 5209K->5209K(9216K), 0.0000165 secs][Tenured: 5587K->5587K(10240K), 0.0029173 secs] 10796K->10713K(19456K), [Metaspace: 3270K->3270K(1056768K)], 0.0029714 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[Full GC (Allocation Failure) [Tenured: 5587K->5527K(10240K), 0.0036178 secs] 10713K->10653K(19456K), [Metaspace: 3270K->3270K(1056768K)], 0.0036448 secs] [Times: user=0.01 sys=0.00, real=0.01 secs]
Heap
 def new generation   total 9216K, used 5508K [0x00000007bec00000, 0x00000007bf600000, 0x00000007bf600000)
  eden space 8192K,  67% used [0x00000007bec00000, 0x00000007bf161370, 0x00000007bf400000)
  from space 1024K,   0% used [0x00000007bf500000, 0x00000007bf500000, 0x00000007bf600000)
  to   space 1024K,   0% used [0x00000007bf400000, 0x00000007bf400000, 0x00000007bf500000)
 tenured generation   total 10240K, used 5527K [0x00000007bf600000, 0x00000007c0000000, 0x00000007c0000000)
   the space 10240K,  53% used [0x00000007bf600000, 0x00000007bfb65f68, 0x00000007bfb66000, 0x00000007c0000000)
 Metaspace       used 3330K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 370K, capacity 388K, committed 512K, reserved 1048576K
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

运行过程

System.gc()

对应的GC日志

1	[Full GC (System.gc()) [Tenured: 0K->467K(10240K), 0.0037172 secs] 2217K->467K(19456K), [Metaspace: 3177K->3177K(1056768K)], 0.0037645 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

进行Full GC的原因是代码调用System.gc()
堆总大小为19456K ≈ 20M
老年代（Tenured Generation）总大小为10240K = 10M，目前占用467K，可忽略
新生代（Def New Generation）目前占用467K-467K=0K，总大小 = 堆大小 - 老年代大小 ≈ 10M，SurvivorRatio=8，因此Eden区大小为8M，Survivor区大小为1M

byte[] b1 = new byte[5 * _1MB]

Eden区有8M空闲空间，能完全容纳b1，不会进行GC

byte[] b2 = new byte[5 * _1MB]

对应的GC日志

1	[GC (Allocation Failure) [DefNew: 5447K->4K(9216K), 0.0036997 secs] 5915K->5592K(19456K), 0.0037186 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

1. `Eden`区只有`8M`，此时已经分配了`b1`，最多只剩下`3M`的可用空间，`Serial`采集器采用`Copying`算法，不足以容纳`b2`，触发`Minor GC` 2. 进行`Minor GC`的原因是`Allocation Failure`，即内存分配失败，与上面分析一致 3. 进行`Minor GC`，首先尝试将`Eden`区和`Survivor 0`区中的`存活对象`一起移到`Survivor 1`区，`b1`是强引用，依旧存活，但由于`Survivor 1`区只有`1M`，无法容纳`b1`，尝试将`b1`直接晋升到`Tenured`区 4. 此时`Tenured`有`10M`的可用空间，能容纳`b1`，可以将`b1`晋升到`Tenured`区，并释放`b1`原本在`Eden`区占用的内存空间 5. `Minor GC`结束后，`b2`便能顺利地在`Eden`区进行分配

byte[] b3 = new byte[5 * _1MB]

对应的GC日志

1
2

[GC (Allocation Failure) [DefNew: 5209K->5209K(9216K), 0.0000165 secs][Tenured: 5587K->5587K(10240K), 0.0029173 secs] 10796K->10713K(19456K), [Metaspace: 3270K->3270K(1056768K)], 0.0029714 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[Full GC (Allocation Failure) [Tenured: 5587K->5527K(10240K), 0.0036178 secs] 10713K->10653K(19456K), [Metaspace: 3270K->3270K(1056768K)], 0.0036448 secs] [Times: user=0.01 sys=0.00, real=0.01 secs]

由于Eden区此时最多有3M的可用空间，要分配b3，首先触发一次Minor GC
由于内存空间（Eden、Survivor 0、Tenured）不足，Minor GC也会失败，进而触发Full GC
Tenured区采用Serial Old收集器，Full GC时会尝试采用Mark-Compact算法进行GC，但依旧会失败，进而导致抛出OutOfMemoryError

小结

尽量避免使用**需要连续内存空间的短命大对象**，例如长字符串和基本类型的大数组，因为这会导致Minor GC的时候，这些短命大对象有可能会直接晋升到老年代，进而加大Full GC的频率

对象年龄晋升

对象每经历一次Minor GC，年龄就会增加1，到了一定的阈值(-XX:MaxTenuringThreshold，默认是15)，就会晋升到老年代

代码

/**
 * 校验JVM参数 -XX:MaxTenuringThreshold
 *
 * @author zhongmingmao [email protected]
 */
public class TenuringThreshold {

    private static final int _10MB = 10 * 1024 * 1024;
    private static Pattern BIN_PATTERN = Pattern.compile("\\(((\\d{8}\\s?){4})\\)");

    // JVM Args : -Xms200m -Xmx200m -Xmn100m -XX:SurvivorRatio=8 -XX:MaxTenuringThreshold=3
    // -XX:+UseSerialGC -XX:+PrintGCDetails  -Djol.tryWithSudo=true
    public static void main(String[] args) {

        System.gc();
        byte[] b = new byte[_10MB / 4];
        System.out.println(String.format("Init Address : %s\n", VM.current().addressOf(b)));

        for (int i = 0; i < 6; ++i) {
            // 从i=1开始，每分配一次bytes，就会触发一次Minor GC
            // 当对象b的GCAge < MaxTenuringThreshold -> 在两个Survivor区之间来回移动
            // 当对象b的GCAge >= MaxTenuringThreshold -> 晋升到Tenured区
            byte[] bytes = new byte[5 * _10MB];
            System.out.println(String.format("Address[%s] : %s , GC Age : %s", i,
                    VM.current().addressOf(b),
                    getObjectGCAge(b)));
            System.out.println();
        }
    }

    /**
     * 获取对象的GC年龄<br/>
     * Java对象头结构 See http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/87ee5ee27509/src/share/vm/oops/markOop.hpp
     */
    private static int getObjectGCAge(Object object) {
        /*
        ClassLayout.parseInstance(object).toPrintable()的完整格式
        [B object internals:
         OFFSET  SIZE   TYPE DESCRIPTION                               VALUE
              0     4        (object header)                           19 00 00 00 (00011001 00000000 00000000
              00000000) (25)
              4     4        (object header)                           00 00 00 00 (00000000 00000000 00000000
              00000000) (0)
              8     4        (object header)                           f5 00 00 f8 (11110101 00000000 00000000
              11111000) (-134217483)
             12     4        (object header)                           00 00 28 00 (00000000 00000000 00101000
             00000000) (2621440)
             16 2621440   byte [B.<elements>                             N/A
        Instance size: 2621456 bytes
        Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
        * */
        String line = ClassLayout.parseInstance(object).toPrintable().split("\n")[2];
        Matcher matcher = BIN_PATTERN.matcher(line);
        if (matcher.find()) {
            return Integer.parseInt(matcher.group(1).split("\\s")[0].substring(1, 5), 2);
        }
        return -1;
    }
}

运行结果

[Full GC (System.gc()) [Tenured: 0K->452K(102400K), 0.0032760 secs] 6555K->452K(194560K), [Metaspace: 3198K->3198K(1056768K)], 0.0033038 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Init Address : 33076281344

Address[0] : 33076281344 , GC Age : 0

[GC (Allocation Failure) [DefNew: 70169K->3565K(92160K), 0.0051274 secs] 70622K->4017K(194560K), 0.0051485 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
Address[1] : 33170780832 , GC Age : 1

[GC (Allocation Failure) [DefNew: 57128K->3417K(92160K), 0.0075981 secs] 57580K->3869K(194560K), 0.0076253 secs] [Times: user=0.01 sys=0.01, real=0.00 secs]
Address[2] : 33160295048 , GC Age : 2

[GC (Allocation Failure) [DefNew: 56715K->3417K(92160K), 0.0020149 secs] 57168K->3869K(194560K), 0.0020339 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Address[3] : 33170780808 , GC Age : 3

[GC (Allocation Failure) [DefNew: 56533K->0K(92160K), 0.0052362 secs] 56985K->3870K(194560K), 0.0052590 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
Address[4] : 33181729784 , GC Age : 3

[GC (Allocation Failure) [DefNew: 52773K->0K(92160K), 0.0004461 secs] 56643K->3870K(194560K), 0.0004610 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
Address[5] : 33181729784 , GC Age : 3

Heap
 def new generation   total 92160K, used 53599K [0x00000007b3800000, 0x00000007b9c00000, 0x00000007b9c00000)
  eden space 81920K,  65% used [0x00000007b3800000, 0x00000007b6c57cd8, 0x00000007b8800000)
  from space 10240K,   0% used [0x00000007b9200000, 0x00000007b92000f0, 0x00000007b9c00000)
  to   space 10240K,   0% used [0x00000007b8800000, 0x00000007b8800000, 0x00000007b9200000)
 tenured generation   total 102400K, used 3869K [0x00000007b9c00000, 0x00000007c0000000, 0x00000007c0000000)
   the space 102400K,   3% used [0x00000007b9c00000, 0x00000007b9fc7710, 0x00000007b9fc7800, 0x00000007c0000000)
 Metaspace       used 6569K, capacity 6832K, committed 7040K, reserved 1056768K
  class space    used 765K, capacity 843K, committed 896K, reserved 1048576K

byte[] b = new byte[_10MB / 4]

1	Init Address : 33076281344

尝试分配对象b到**Eden**区，内存空间充足，初始内存地址为33076281344，GC年龄为0

循环`i=0`

1	Address[0] : 33076281344 , GC Age : 0

尝试在Eden区分配5*_10MB的内存空间，内存空间充足，不需要触发Minor GC，对象b的内存地址不变，依旧是33076281344，在**Eden**区，GC年龄为0

循环`i=1`

1 2	[GC (Allocation Failure) [DefNew: 70169K->3565K(92160K), 0.0051274 secs] 70622K->4017K(194560K), 0.0051485 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] Address[1] : 33170780832 , GC Age : 1

尝试在Eden区再分配5*_10MB的内存空间，内存空间不足，触发Minor GC，循环i=0分配的5*_10MB的内存空间被回收，对象b被移动到了**Survivor 0**区，内存地址变成了33170780832，GC年龄为1

循环`i=2`

1 2	[GC (Allocation Failure) [DefNew: 57128K->3417K(92160K), 0.0075981 secs] 57580K->3869K(194560K), 0.0076253 secs] [Times: user=0.01 sys=0.01, real=0.00 secs] Address[2] : 33160295048 , GC Age : 2

尝试在Eden区再分配5*_10MB的内存空间，内存空间不足，触发Minor GC，循环i=1分配的5*_10MB的内存空间被回收，对象b被移动到了**Survivor 1**区，内存地址变成了33160295048，GC年龄为2

循环`i=3`

1 2	[GC (Allocation Failure) [DefNew: 56715K->3417K(92160K), 0.0020149 secs] 57168K->3869K(194560K), 0.0020339 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] Address[3] : 33170780808 , GC Age : 3

尝试在Eden区再分配5*_10MB的内存空间，内存空间不足，触发Minor GC，循环i=2分配的5*_10MB的内存空间被回收，对象b**再次被移动到了Survivor 0**区，内存地址变成了33170780808，GC年龄为3

循环`i=4`

1 2	[GC (Allocation Failure) [DefNew: 56533K->0K(92160K), 0.0052362 secs] 56985K->3870K(194560K), 0.0052590 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] Address[4] : 33181729784 , GC Age : 3

尝试在Eden区再分配5*_10MB的内存空间，内存空间不足，触发Minor GC，循环i=3分配的5*_10MB的内存空间被回收，对象b的GC年龄已经达到了MaxTenuringThreshold，可以直接晋升到Tenured区，内存地址变成了33181729784，GC年龄依旧为3,不会再增加

循环`i=5`

1 2	[GC (Allocation Failure) [DefNew: 52773K->0K(92160K), 0.0004461 secs] 56643K->3870K(194560K), 0.0004610 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] Address[5] : 33181729784 , GC Age : 3

尝试在Eden区再分配5*_10MB的内存空间，内存空间不足，触发Minor GC，循环i=4分配的5*_10MB的内存空间被回收，对象b已经在Tenured区，此时并不是Full GC，内存地址不会改变，依旧是33181729784，GC年龄也依旧是3

动态晋升

在Minor GC时，如果在Survivor中**相同年龄的所有对象大小之和** ≧ 0.5 * sizeof(Survivor) ⇒ **大于或等于该年龄的对象直接晋升**到老年代，无须考虑MaxTenuringThreshold

代码

/**
 * 验证动态晋升<br/>
 *
 * @author zhongmingmao zhongmingmao@[email protected]
 */
public class DynamicGCAge {

    private static final int _10MB = 10 * 1024 * 1024;
    private static Pattern BIN_PATTERN = Pattern.compile("\\(((\\d{8}\\s?){4})\\)");

    // JVM Args : -Xms200m -Xmx200m -Xmn100m -XX:SurvivorRatio=8 -XX:+UseSerialGC
    // -XX:+PrintGCDetails -Djol.tryWithSudo=true
    public static void main(String[] args) {
        System.gc();
        byte[] b0 = new byte[_10MB / 8];
        byte[] b1 = null;
        byte[] b2 = null;
        byte[] b3 = null;
        byte[] b4 = null;
        System.out.println(getObjectInfo("b0", b0));
        System.out.println();
        for (int i = 0; i < 5; ++i) {
            // i=2才实例化b1~b4,b1的GC年龄比b1~b4大，且b1~b4的总大小等于Survivor区的一半，下次Minor GC，能直接晋升
            if (i == 2) {
                b1 = new byte[_10MB / 8];
                b2 = new byte[_10MB / 8];
                b3 = new byte[_10MB / 8];
                b4 = new byte[_10MB / 8];
            }
            byte[] bytes = new byte[5 * _10MB];

            System.out.println(getObjectInfo("b0", b0));
            System.out.println(getObjectInfo("b1", b1));
            System.out.println(getObjectInfo("b2", b2));
            System.out.println(getObjectInfo("b3", b3));
            System.out.println(getObjectInfo("b4", b4));
            System.out.println();
        }
    }

    private static int getObjectGCAge(Object object) {
        if (null == object) {
            return -1;
        }
        String line = ClassLayout.parseInstance(object).toPrintable().split("\n")[2];
        Matcher matcher = BIN_PATTERN.matcher(line);
        if (matcher.find()) {
            return Integer.parseInt(matcher.group(1).split("\\s")[0].substring(1, 5), 2);
        }
        return 0;
    }

    private static long getObjectAddress(Object object) {
        return null == object ? -1 : VM.current().addressOf(object);
    }

    private static String getObjectInfo(String name, Object object) {
        return String.format("Obj[%s] , Address:[%s] , GCAge:[%s]",
                name,
                getObjectAddress(object),
                getObjectGCAge(object));
    }
}

运行结果

[Full GC (System.gc()) [Tenured: 0K->484K(102400K), 0.0046554 secs] 6555K->484K(194560K), [Metaspace: 3346K->3346K(1056768K)], 0.0047017 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
Obj[b0] , Address:[33076281344] , GCAge:[0]

Obj[b0] , Address:[33076281344] , GCAge:[0]
Obj[b1] , Address:[-1] , GCAge:[-1]
Obj[b2] , Address:[-1] , GCAge:[-1]
Obj[b3] , Address:[-1] , GCAge:[-1]
Obj[b4] , Address:[-1] , GCAge:[-1]

[GC (Allocation Failure) [DefNew: 67248K->2254K(92160K), 0.0069139 secs] 67732K->2738K(194560K), 0.0069444 secs] [Times: user=0.00 sys=0.01, real=0.01 secs]
Obj[b0] , Address:[33170752344] , GCAge:[1]
Obj[b1] , Address:[-1] , GCAge:[-1]
Obj[b2] , Address:[-1] , GCAge:[-1]
Obj[b3] , Address:[-1] , GCAge:[-1]
Obj[b4] , Address:[-1] , GCAge:[-1]

[GC (Allocation Failure) [DefNew: 59673K->7226K(92160K), 0.0059408 secs] 60157K->7710K(194560K), 0.0059641 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
Obj[b0] , Address:[33160266584] , GCAge:[2]
Obj[b1] , Address:[33161577320] , GCAge:[1]
Obj[b2] , Address:[33162888056] , GCAge:[1]
Obj[b3] , Address:[33164198792] , GCAge:[1]
Obj[b4] , Address:[33165509528] , GCAge:[1]

[GC (Allocation Failure) [DefNew: 60004K->0K(92160K), 0.0086877 secs] 60488K->7710K(194560K), 0.0087102 secs] [Times: user=0.00 sys=0.01, real=0.00 secs]
Obj[b0] , Address:[33181733760] , GCAge:[2]
Obj[b1] , Address:[33183044496] , GCAge:[1]
Obj[b2] , Address:[33184355232] , GCAge:[1]
Obj[b3] , Address:[33185665968] , GCAge:[1]
Obj[b4] , Address:[33186976704] , GCAge:[1]

[GC (Allocation Failure) [DefNew: 53328K->0K(92160K), 0.0009302 secs] 61039K->7710K(194560K), 0.0009761 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Obj[b0] , Address:[33181733760] , GCAge:[2]
Obj[b1] , Address:[33183044496] , GCAge:[1]
Obj[b2] , Address:[33184355232] , GCAge:[1]
Obj[b3] , Address:[33185665968] , GCAge:[1]
Obj[b4] , Address:[33186976704] , GCAge:[1]

Heap
 def new generation   total 92160K, used 53610K [0x00000007b3800000, 0x00000007b9c00000, 0x00000007b9c00000)
  eden space 81920K,  65% used [0x00000007b3800000, 0x00000007b6c5aa18, 0x00000007b8800000)
  from space 10240K,   0% used [0x00000007b8800000, 0x00000007b8800050, 0x00000007b9200000)
  to   space 10240K,   0% used [0x00000007b9200000, 0x00000007b9200000, 0x00000007b9c00000)
 tenured generation   total 102400K, used 7710K [0x00000007b9c00000, 0x00000007c0000000, 0x00000007c0000000)
   the space 102400K,   7% used [0x00000007b9c00000, 0x00000007ba387ae8, 0x00000007ba387c00, 0x00000007c0000000)
 Metaspace       used 6608K, capacity 6832K, committed 7040K, reserved 1056768K
  class space    used 765K, capacity 843K, committed 896K, reserved 1048576K

byte[] b0 = new byte[_10MB / 8]

1	Obj[b0] , Address:[33076281344] , GCAge:[0]

尝试分配对象b0到**Eden**区，内存空间充足，初始内存地址为33076281344，GC年龄为0

循环`i=0`

Obj[b0] , Address:[33076281344] , GCAge:[0]
Obj[b1] , Address:[-1] , GCAge:[-1]
Obj[b2] , Address:[-1] , GCAge:[-1]
Obj[b3] , Address:[-1] , GCAge:[-1]
Obj[b4] , Address:[-1] , GCAge:[-1]

尝试在Eden区分配5*_10MB的内存空间，内存空间充足，不需要触发Minor GC，对象b0的内存地址不变，依旧是33076281344，在**Eden**区，GC年龄为0
b1~b4尚未实例化

循环`i=1`

[GC (Allocation Failure) [DefNew: 67248K->2254K(92160K), 0.0069139 secs] 67732K->2738K(194560K), 0.0069444 secs] [Times: user=0.00 sys=0.01, real=0.01 secs]
Obj[b0] , Address:[33170752344] , GCAge:[1]
Obj[b1] , Address:[-1] , GCAge:[-1]
Obj[b2] , Address:[-1] , GCAge:[-1]
Obj[b3] , Address:[-1] , GCAge:[-1]
Obj[b4] , Address:[-1] , GCAge:[-1]

尝试在Eden区再分配5*_10MB的内存空间，内存空间不足，触发Minor GC，循环i=0分配的5*_10MB的内存空间被回收，对象b0被移动到了**Survivor 0**区，内存地址变成了33170752344，GC年龄为1
b1~b4尚未实例化

循环`i=2`

[GC (Allocation Failure) [DefNew: 59673K->7226K(92160K), 0.0059408 secs] 60157K->7710K(194560K), 0.0059641 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
Obj[b0] , Address:[33160266584] , GCAge:[2]
Obj[b1] , Address:[33161577320] , GCAge:[1]
Obj[b2] , Address:[33162888056] , GCAge:[1]
Obj[b3] , Address:[33164198792] , GCAge:[1]
Obj[b4] , Address:[33165509528] , GCAge:[1]

尝试在Eden区再分配b1~b4的内存空间，内存空间充足，不会触发Minor GC
尝试在Eden区再分配5*_10MB的内存空间，内存空间不足，触发Minor GC，循环i=1分配的5*_10MB的内存空间被回收，对象b0被移动到了**Survivor 1**区，内存地址变成了33160266584，GC年龄为2
另外这次Minor GC也把b1~b4移动到了**Survivor 1**区，具体信息GC日志所示

循环`i=3`

[GC (Allocation Failure) [DefNew: 60004K->0K(92160K), 0.0086877 secs] 60488K->7710K(194560K), 0.0087102 secs] [Times: user=0.00 sys=0.01, real=0.00 secs]
Obj[b0] , Address:[33181733760] , GCAge:[2]
Obj[b1] , Address:[33183044496] , GCAge:[1]
Obj[b2] , Address:[33184355232] , GCAge:[1]
Obj[b3] , Address:[33185665968] , GCAge:[1]
Obj[b4] , Address:[33186976704] , GCAge:[1]

尝试在Eden区再分配5*_10MB的内存空间，内存空间不足，触发Minor GC，循环i=2分配的5*_10MB的内存空间被回收，此时**Survivor 1区中b1~b4具有相同的GC年龄，总大小 = 5M = 0.5 * sizeof(Survivor)，可以进行动态晋升**，所有GC年龄大于等于1的对象都可以直接晋升到老年代，因此b0~b4直接晋升，具体信息GC日志所示

循环`i=4`

[GC (Allocation Failure) [DefNew: 53328K->0K(92160K), 0.0009302 secs] 61039K->7710K(194560K), 0.0009761 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Obj[b0] , Address:[33181733760] , GCAge:[2]
Obj[b1] , Address:[33183044496] , GCAge:[1]
Obj[b2] , Address:[33184355232] , GCAge:[1]
Obj[b3] , Address:[33185665968] , GCAge:[1]
Obj[b4] , Address:[33186976704] , GCAge:[1]

尝试在Eden区再分配5*_10MB的内存空间，内存空间不足，触发Minor GC，循环i=3分配的5*_10MB的内存空间被回收，此时b0~b4都已经在老年代中了，此时只是Minor GC，内存地址和GC年龄都不会改变

-XX:+UseSerialGC

Minor GC / Major GC / Full GC

常规理解

Minor GC

Major GC / Full GC

大对象直接晋升

代码

运行结果

运行过程

System.gc()

byte[] b1 = new byte[5 * _1MB]

byte[] b2 = new byte[5 * _1MB]

byte[] b3 = new byte[5 * _1MB]

小结

对象年龄晋升

代码

运行结果

byte[] b = new byte[_10MB / 4]

循环i=0

循环i=1

循环i=2

循环i=3

循环i=4

循环i=5

动态晋升

代码

运行结果

byte[] b0 = new byte[_10MB / 8]

循环i=0

循环i=1

循环i=2

循环i=3

循环i=4

循环`i=0`

循环`i=1`

循环`i=2`

循环`i=3`

循环`i=4`

循环`i=5`

循环`i=0`

循环`i=1`

循环`i=2`

循环`i=3`

循环`i=4`