深入理解java虚拟机（六）：java垃圾收集分析实战（内存分配与回收策略）
深入理解java虚拟机（一）：java内存区域（内存结构划分）
深入理解java虚拟机（二）：java内存溢出实战
深入理解java虚拟机（三）：String.intern()-字符串常量池
深入理解java虚拟机（四）：对象存活判定算法和垃圾收集算法
深入理解java虚拟机（五）：hotspot垃圾收集算法实现
深入理解java虚拟机（六）：java垃圾收集分析实战（内存分配与回收策略）
深入理解java虚拟机（七）：java垃圾收集分析总结
深入理解java虚拟机（八）：java内存分析工具-MAT和OQL
试验环境jdk 1.6.0_37。
虚拟机参数-XX:PrintGCDetails让虚拟机在发生垃圾回收行为时打印内存回收日志，并在进程退出时候输出当前的内存各区域分配情况。
以下代码测试都将加上 -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 参数，即设置java堆大小限制为20M,新生代10M，Eden区和Survivor区的
比例是8，即Eden区为8M，每个Survivor区大小为1M，并打印垃圾回收日志。
一、对象优先在Eden分配
在大多数情况下，对象优先分配在Eden区，当Eden区没有足够的空间进行分配时，虚拟机将发起一次Minor GC。
/**
* VM参数：-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8
* 限制java堆大小为20M，年轻代为10M，Eden为8M，两个survivor分别为1M
*/
public static void testAllocation() {
byte[] allocation1, allocation2, allocation3, allocation4;
allocation1 = new byte[2 * _1MB];
allocation2 = new byte[2 * _1MB];
allocation3 = new byte[2 * _1MB];
allocation4 = new byte[4 * _1MB]; // 出现一次Minor GC
/**
[GC [DefNew: 6487K->152K(9216K), 0.0040116 secs] 6487K->6296K(19456K), 0.0040436 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
def new generation total 9216K, used 4576K [0x32750000, 0x33150000, 0x33150000)
eden space 8192K, 54% used [0x32750000, 0x32ba1fa8, 0x32f50000)
from space 1024K, 14% used [0x33050000, 0x33076150, 0x33150000)
to space 1024K, 0% used [0x32f50000, 0x32f50000, 0x33050000)
tenured generation total 10240K, used 6144K [0x33150000, 0x33b50000, 0x33b50000)
the space 10240K, 60% used [0x33150000, 0x33750030, 0x33750200, 0x33b50000)
compacting perm gen total 12288K, used 376K [0x33b50000, 0x34750000, 0x37b50000)
the space 12288K, 3% used [0x33b50000, 0x33bae2c0, 0x33bae400, 0x34750000)
ro space 10240K, 55% used [0x37b50000, 0x380d1140, 0x380d1200, 0x38550000)
rw space 12288K, 55% used [0x38550000, 0x38bf44c8, 0x38bf4600, 0x39150000)
*
*/
}
二、大对象直接进入老年代
当创建的对象超过指定大小时，直接把对象分配在老年代中。
-XX:PretenureSizeThreshold=3145728 参数设定超过对象超过多少时，分配到老年代中，此例为3M（3*1024*1024）。
/**
* VM参数：-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:PretenureSizeThreshold=3145728
* 对象超过3M 时直接进入老年代
*/
public static void testPretenureSizeThreshold() {
byte[] allocation;
allocation = new byte[4 * _1MB]; //直接分配在老年代中
/**
Heap
def new generation total 9216K, used 507K [0x32750000, 0x33150000, 0x33150000)
eden space 8192K, 6% used [0x32750000, 0x327cef38, 0x32f50000)
from space 1024K, 0% used [0x32f50000, 0x32f50000, 0x33050000)
to space 1024K, 0% used [0x33050000, 0x33050000, 0x33150000)
tenured generation total 10240K, used 4096K [0x33150000, 0x33b50000, 0x33b50000)
the space 10240K, 40% used [0x33150000, 0x33550010, 0x33550200, 0x33b50000)
compacting perm gen total 12288K, used 376K [0x33b50000, 0x34750000, 0x37b50000)
the space 12288K, 3% used [0x33b50000, 0x33bae3b8, 0x33bae400, 0x34750000)
ro space 10240K, 55% used [0x37b50000, 0x380d1140, 0x380d1200, 0x38550000)
rw space 12288K, 55% used [0x38550000, 0x38bf44c8, 0x38bf4600, 0x39150000)
*/
}
三、长期存活对象进入老年代
对象在两个survivor区每复制（Minor gc）一次，年龄就增长一岁，当超过指定最大随时时转移到老年代中。
-XX:MaxTenuringThreshold=8 参数用于设定对象最大年龄阈值。
/**
* VM参数：-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:MaxTenuringThreshold=1 -XX:+PrintTenuringDistribution
*
*/
@SuppressWarnings("unused")
public static void testTenuringThreshold() {
byte[] allocation1, allocation2, allocation3;
allocation1 = new byte[_1MB / 4]; //262144 什么时候进入老年代决定于XX:MaxTenuringThreshold设置
allocation2 = new byte[4 * _1MB]; //4194304
allocation3 = new byte[4 * _1MB];
allocation3 = null;
allocation3 = new byte[4 * _1MB];
/**
*
* XX:MaxTenuringThreshold =8
[GC [DefNew
Desired survivor size 524288 bytes, new threshold 8 (max 8)
- age 1: 418144 bytes, 418144 total
: 4695K->408K(9216K), 0.0036693 secs] 4695K->4504K(19456K), 0.0036983 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC [DefNew
Desired survivor size 524288 bytes, new threshold 8 (max 8)
- age 1: 136 bytes, 136 total
- age 2: 417936 bytes, 418072 total
: 4668K->408K(9216K), 0.0010034 secs] 8764K->4504K(19456K), 0.0010296 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
def new generation total 9216K, used 4668K [0x32750000, 0x33150000, 0x33150000)
eden space 8192K, 52% used [0x32750000, 0x32b78fe0, 0x32f50000)
from space 1024K, 39% used [0x32f50000, 0x32fb6118, 0x33050000)
to space 1024K, 0% used [0x33050000, 0x33050000, 0x33150000)
tenured generation total 10240K, used 4096K [0x33150000, 0x33b50000, 0x33b50000)
the space 10240K, 40% used [0x33150000, 0x33550010, 0x33550200, 0x33b50000)
compacting perm gen total 12288K, used 377K [0x33b50000, 0x34750000, 0x37b50000)
the space 12288K, 3% used [0x33b50000, 0x33bae5b8, 0x33bae600, 0x34750000)
ro space 10240K, 55% used [0x37b50000, 0x380d1140, 0x380d1200, 0x38550000)
rw space 12288K, 55% used [0x38550000, 0x38bf44c8, 0x38bf4600, 0x39150000)
*/
/**
*
* XX:MaxTenuringThreshold=1
[GC [DefNew
Desired survivor size 524288 bytes, new threshold 1 (max 1)
- age 1: 418144 bytes, 418144 total
: 4695K->408K(9216K), 0.0054252 secs] 4695K->4504K(19456K), 0.0054708 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]
[GC [DefNew
Desired survivor size 524288 bytes, new threshold 1 (max 1)
- age 1: 136 bytes, 136 total
: 4668K->0K(9216K), 0.0013601 secs] 8764K->4504K(19456K), 0.0013867 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
def new generation total 9216K, used 4260K [0x32750000, 0x33150000, 0x33150000)
eden space 8192K, 52% used [0x32750000, 0x32b78fe0, 0x32f50000)
from space 1024K, 0% used [0x32f50000, 0x32f50088, 0x33050000)
to space 1024K, 0% used [0x33050000, 0x33050000, 0x33150000)
tenured generation total 10240K, used 4504K [0x33150000, 0x33b50000, 0x33b50000)
the space 10240K, 43% used [0x33150000, 0x335b60a0, 0x335b6200, 0x33b50000)
compacting perm gen total 12288K, used 377K [0x33b50000, 0x34750000, 0x37b50000)
the space 12288K, 3% used [0x33b50000, 0x33bae5c0, 0x33bae600, 0x34750000)
ro space 10240K, 55% used [0x37b50000, 0x380d1140, 0x380d1200, 0x38550000)
rw space 12288K, 55% used [0x38550000, 0x38bf44c8, 0x38bf4600, 0x39150000)
*/
}
解释
- age 1: 136 bytes, 136 total- age 2: 417936 bytes, 418072 total
- age 2: 417936 bytes, 418072 total
表示 年龄为1 的对象有总共多大，以此类似。total 从上往下加。
四、对象动态年龄判断
虽然可以设置对象最大年龄阈值，但有时候虚拟机会根据内存情况自己动态计算对象的阈值。
当survivor区相同年龄的大小不小于survivor区的一半时，虚拟机会把survivor区等于和大于此年龄的对象转移到老年代。
/**
* VM参数：-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:MaxTenuringThreshold=15 -XX:+PrintTenuringDistribution
*
*/
@SuppressWarnings("unused")
public static void testTenuringThreshold2() {
byte[] allocation1, allocation2, allocation3, allocation4;
allocation1 = new byte[_1MB / 4]; // allocation1+allocation2大于survivo空间一半
allocation2 = new byte[_1MB / 4];
allocation3 = new byte[4 * _1MB];
allocation4 = new byte[4 * _1MB];
allocation4 = null;
allocation4 = new byte[4 * _1MB];
/**
* [GC [DefNew
Desired survivor size 524288 bytes, new threshold 1 (max 15)
- age 1: 680304 bytes, 680304 total
: 4951K->664K(9216K), 0.0033210 secs] 4951K->4760K(19456K), 0.0033442 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC [DefNew
Desired survivor size 524288 bytes, new threshold 15 (max 15)
- age 1: 136 bytes, 136 total
: 4924K->0K(9216K), 0.0011772 secs] 9020K->4760K(19456K), 0.0011987 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
def new generation total 9216K, used 4260K [0x32750000, 0x33150000, 0x33150000)
eden space 8192K, 52% used [0x32750000, 0x32b78fe0, 0x32f50000)
from space 1024K, 0% used [0x32f50000, 0x32f50088, 0x33050000)
to space 1024K, 0% used [0x33050000, 0x33050000, 0x33150000)
tenured generation total 10240K, used 4760K [0x33150000, 0x33b50000, 0x33b50000)
the space 10240K, 46% used [0x33150000, 0x335f60b0, 0x335f6200, 0x33b50000)
compacting perm gen total 12288K, used 377K [0x33b50000, 0x34750000, 0x37b50000)
the space 12288K, 3% used [0x33b50000, 0x33bae5c0, 0x33bae600, 0x34750000)
ro space 10240K, 55% used [0x37b50000, 0x380d1140, 0x380d1200, 0x38550000)
rw space 12288K, 55% used [0x38550000, 0x38bf44c8, 0x38bf4600, 0x39150000)
*/
}
解释：
new threshold 1 (max 15) 表示 虚拟机自己计算的阈值为1 ，最大15。
五、空间分配担保
在发送minor gc之前，虚拟机会首先检查老年代最大可连续空间是否大于新生代所有对象总和，如果这个条件成立，可以确保这次minor gc是安全的，
如果不成立，虚拟机会查看HandlePromotionFailure设置值是否允许担保失败。如果允许，那么会继续检查老年代最大可连续空间是否大于历次晋升到老年代对象
的评价大小，如果大于，将尝试一次minor gc，尽管这次minor gc是有风险的；如果小于，或者HandlePromotionFailure设置不允许冒险，那么这时也要改为一次Full gc。
在jdk1.6 update24之后，HandlePromotionFailure参数不会影响虚拟机空间分配担保策略，虚拟机改为，只要老年代最大连续空间大于新生代对象总和或者大于历次晋升平均大小，都将进行minor gc，否则将进行Full gc。
/**
* VM参数：-Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:-HandlePromotionFailure
*/
@SuppressWarnings("unused")
public static void testHandlePromotion() {
byte[] allocation1, allocation2, allocation3, allocation4, allocation5, allocation6, allocation7;
allocation1 = new byte[2 * _1MB];
allocation2 = new byte[2 * _1MB];
allocation3 = new byte[2 * _1MB];
allocation1 = null;
allocation4 = new byte[2 * _1MB];
allocation5 = new byte[2 * _1MB];
allocation6 = new byte[2 * _1MB];
allocation4 = null;
allocation5 = null;
allocation6 = null;
allocation7 = new byte[2 * _1MB];
/**
*
java.version = 1.6.0_37
[GC [DefNew: 6487K->152K(9216K), 0.0040346 secs] 6487K->4248K(19456K), 0.0040639 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC [DefNew: 6546K->152K(9216K), 0.0004896 secs] 10642K->4248K(19456K), 0.0005141 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
def new generation total 9216K, used 2364K [0x32750000, 0x33150000, 0x33150000)
eden space 8192K, 27% used [0x32750000, 0x32978fe0, 0x32f50000)
from space 1024K, 14% used [0x32f50000, 0x32f76108, 0x33050000)
to space 1024K, 0% used [0x33050000, 0x33050000, 0x33150000)
tenured generation total 10240K, used 4096K [0x33150000, 0x33b50000, 0x33b50000)
the space 10240K, 40% used [0x33150000, 0x33550020, 0x33550200, 0x33b50000)
compacting perm gen total 12288K, used 377K [0x33b50000, 0x34750000, 0x37b50000)
the space 12288K, 3% used [0x33b50000, 0x33bae758, 0x33bae800, 0x34750000)
ro space 10240K, 55% used [0x37b50000, 0x380d1140, 0x380d1200, 0x38550000)
rw space 12288K, 55% used [0x38550000, 0x38bf44c8, 0x38bf4600, 0x39150000)
Warning: The flag -HandlePromotionFailure has been EOL'd as of 6.0_24 and will be ignored
*/
}
