什么是fullGC：
从年轻代空间（包括 Eden 和 Survivor 区域）回收内存被称为 Minor GC，对老年代GC称为Major GC,而Full GC是对整个堆来说的，在最近几个版本的JDK里默认包括了对永久代即方法区的回收（JDK8中无永久带了），出现Full GC的时候经常伴随至少一次的Minor GC,但非绝对的。
会引发fullGC的几种情况：https://blog.csdn.net/qq_38384440/article/details/81710887
分析的话需要从JVM内存结构说起（根据JDK版本会有所调整）：
a、JVM内存模型(图片来自网络)：
b、我们主要看一下堆的构成(图片来自网络)：
1.JVM中堆空间可以分成三个大区，新生代、老年代、永久代
2.新生代可以划分为三个区，Eden区，两个幸存区
c、生存情况(图片来自网络)：
定位问题：
工具：JDK自带的Jmap等工具：%JAVA_HOME%/bin/jmap
1、查看JVM参数命令：ps -ef |grep java
2、top命令查看PID
1、top命令查看PID
>top
空闲：
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
159910 admin 20 0 6593m 2.2g 16m S 3.7 27.4 2:32.04 java
2270 root 20 0 296m 25m 6792 S 1.3 0.3 0:04.93 python
普通任务：
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
159910 admin 20 0 6725m 2.9g 9452 S 20.6 35.8 59:38.10 java
644 root 20 0 238m 4356 3168 S 1.7 0.1 27:24.38 alisentry_cli
多线程任务：
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
159910 admin 20 0 6818m 4.4g 9648 S 182.7 54.9 74:45.76 java
644 root 20 0 238m 4356 3168 S 0.8 0.1 27:30.43 alisentry_cli
3、jmap命令
注意[B 对象 ，这是一个byte数组，可以dump内存中的数据查看内容，会发现实际上就是传输的表数据
>sudo -u admin /opt/taobao/java/bin/jmap -histo:live 159910 | more
num #instances #bytes class name
空闲：
1: 16391 110613520 [Ljava.lang.Object;
2: 108702 31890616 [C
3: 8075 12048648 [B
普通任务：
1: 6740 163115880 [B
2: 16773 110638024 [Ljava.lang.Object;
3: 110747 32302984 [C
多线程任务开启：
1: 9468 2065856344 [B
2: 18254 110830840 [Ljava.lang.Object;
3: 130436 34780696 [C
查看堆栈信息：
启动任务后：
$sudo -u admin /opt/taobao/java/bin/jmap -heap 159910
Attaching to process ID 2438, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.66-b60
using parallel threads in the new generation.
using thread-local object allocation.
Concurrent Mark-Sweep GC
Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 4294967296 (4096.0MB)
NewSize = 2147483648 (2048.0MB)
MaxNewSize = 2147483648 (2048.0MB)
OldSize = 2147483648 (2048.0MB)
NewRatio = 2
SurvivorRatio = 10
MetaspaceSize = 268435456 (256.0MB)
CompressedClassSpaceSize = 1073741824 (1024.0MB)
MaxMetaspaceSize = 268435456 (256.0MB)
G1HeapRegionSize = 0 (0.0MB)
Heap Usage:
New Generation (Eden + 1 Survivor Space):新生代区内存分布，包含伊甸园区+Survivor区
capacity = 1968570368 (1877.375MB)
used = 1965707016 (1874.6442947387695MB)
free = 2863352 (2.7307052612304688MB)
99.85454662700683% used
Eden Space:Eden区内存分布
capacity = 1789657088 (1706.75MB)
used = 1788720872 (1705.8571548461914MB)
free = 936216 (0.8928451538085938MB)
99.94768740859477% used
From Space（S1）:其中一个Survivor区的内存分布
capacity = 178913280 (170.625MB)
used = 176986144 (168.78713989257812MB)
free = 1927136 (1.837860107421875MB)
98.92286587110806% used
To Space（S2）:另一个Survivor区的内存分布
capacity = 178913280 (170.625MB)
used = 0 (0.0MB)
free = 178913280 (170.625MB)
0.0% used
concurrent mark-sweep generation（永久代，fullGC位置）:
capacity = 2147483648 (2048.0MB)
used = 2147235096 (2047.7629623413086MB)
free = 248552 (0.23703765869140625MB)
99.98842589557171% used
32663 interned Strings occupying 3788288 bytes.查找定位堆栈线程异常：
FULL GC 详情：sudo -u admin /opt/taobao/java/bin/jstack 159910参考地址：
http://www.cnblogs.com/myna/p/7573843.html
https://www.cnblogs.com/kongzhongqijing/articles/3621163.html
top命令，查看java PID 为29037
ps -ef | grep java 查看java进程及相关参数
jMap：
sudo -u admin /opt/taobao/java/bin/jmap -heap 29037
sudo -u admin /opt/taobao/java/bin/jmap -histo:live 29037 | more
sudo -u admin /opt/taobao/java/bin/jmap -heap 29037
sudo -u admin /opt/taobao/java/bin/jmap -dump:format=b,file=/home/admin/mem.dat 29037
sudo -u admin /opt/taobao/java/bin/jstack 29037
