一、创建会内存溢出的程序

pom：

4.0.0com.fanJVMTest1.0-SNAPSHOTorg.apache.maven.pluginsmaven-shade-plugin3.2.0packageshade{Mainclass}com.fan.T15_Fu11GC_Problem01org.apache.maven.pluginsmaven-compiler-plugin88jarJVMTesthttp://maven.apache.orgUTF-8junitjunit3.8.1testorg.apache.tomcat.embedtomcat-embed-core9.0.58backport-util-concurrentbackport-util-concurrent3.1

Java：

package com.fan;import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;public class T15_Fu11GC_Problem01 {private static ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(50, new ThreadPoolExecutor.DiscardOldestPolicy());public static void main(String[] args) throws Exception {executor.setMaximumPoolSize(50);for (; ; ) {modelFit();Thread.sleep(100);}}private static class CardInfo {BigDecimal price = new BigDecimal(0.0);String name = "张三";int age = 5;Date birihdate = new Date();public void m() {}}private static void modelFit() {List taskList = getAllCardInfo();taskList.forEach(info -> {executor.scheduleWithFixedDelay(() -> {info.m();}, 2, 3, TimeUnit.SECONDS);});}private static List getAllCardInfo() {List taskList = new ArrayList<>();for (int i = 0; i < 100; i++) {CardInfo ci = new CardInfo();taskList.add(ci);}return taskList;}
}

打包 jar：

 将 JVMTest-1.0-SNAPSHOT.jar 包放到服务器中 

 二、启动 JVMTest-1.0-SNAPSHOT.jar

java -Xms200M -Xmx200M -XX:+PrintGC -jar JVMTest-1.0-SNAPSHOT.jar 

如图，随着时间的推移，GC每隔一段时间回收一次。

jvm排查常用命令： 

1. jps：该命令会显示所有的 java进程，这里我们的测试程序为 4869 jar

2. jinfo 进程号： 该命令显示 对应进程号进程的相关信息

[root@centos142 arthas]# jinfo 4869
Attaching to process ID 4869, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.333-b02
Java System Properties:java.runtime.name = Java(TM) SE Runtime Environment
java.vm.version = 25.333-b02
sun.boot.library.path = /usr/local/jdk1.8/jre/lib/amd64
java.vendor.url = http://java.oracle.com/
java.vm.vendor = Oracle Corporation
path.separator = :
file.encoding.pkg = sun.io
java.vm.name = Java HotSpot(TM) 64-Bit Server VM
sun.os.patch.level = unknown
sun.java.launcher = SUN_STANDARD
user.country = CN
user.dir = /mnt/arthas
java.vm.specification.name = Java Virtual Machine Specification
java.runtime.version = 1.8.0_333-b02
java.awt.graphicsenv = sun.awt.X11GraphicsEnvironment
os.arch = amd64
java.endorsed.dirs = /usr/local/jdk1.8/jre/lib/endorsed
java.io.tmpdir = /tmp
line.separator = java.vm.specification.vendor = Oracle Corporation
os.name = Linux
sun.jnu.encoding = UTF-8
java.library.path = /usr/java/packages/lib/amd64:/usr/lib64:/lib64:/lib:/usr/lib
java.specification.name = Java Platform API Specification
java.class.version = 52.0
sun.management.compiler = HotSpot 64-Bit Tiered Compilers
os.version = 3.10.0-1160.el7.x86_64
user.home = /root
user.timezone = 
java.awt.printerjob = sun.print.PSPrinterJob
file.encoding = UTF-8
java.specification.version = 1.8
user.name = root
java.class.path = JVMTest-1.0-SNAPSHOT.jar
java.vm.specification.version = 1.8
sun.arch.data.model = 64
sun.java.command = JVMTest-1.0-SNAPSHOT.jar
java.home = /usr/local/jdk1.8/jre
user.language = zh
java.specification.vendor = Oracle Corporation
awt.toolkit = sun.awt.X11.XToolkit
java.vm.info = mixed mode
java.version = 1.8.0_333
java.ext.dirs = /usr/local/jdk1.8/jre/lib/ext:/usr/java/packages/lib/ext
sun.boot.class.path = /usr/local/jdk1.8/jre/lib/resources.jar:/usr/local/jdk1.8/jre/lib/rt.jar:/usr/local/jdk1.8/jre/lib/sunrsasign.jar:/usr/local/jdk1.8/jre/lib/jsse.jar:/usr/local/jdk1.8/jre/lib/jce.jar:/usr/local/jdk1.8/jre/lib/charsets.jar:/usr/local/jdk1.8/jre/lib/jfr.jar:/usr/local/jdk1.8/jre/classes
java.vendor = Oracle Corporation
file.separator = /
java.vendor.url.bug = http://bugreport.sun.com/bugreport/
sun.io.unicode.encoding = UnicodeLittle
sun.cpu.endian = little
sun.cpu.isalist = VM Flags:
Non-default VM flags: -XX:CICompilerCount=2 -XX:InitialHeapSize=209715200 -XX:MaxHeapSize=209715200 -XX:MaxNewSize=69730304 -XX:MinHeapDeltaBytes=524288 -XX:NewSize=69730304 -XX:OldSize=139984896 -XX:+PrintGC -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseFastUnorderedTimeStamps -XX:+UseParallelGC 
Command line:  -Xms200M -Xmx200M -XX:+PrintGC

 例如，这里我们启动的程序使用的是  -Xms200M -Xmx200M -XX:+PrintGC

3.  jstat  -[gc] 进程号 [500]：该命令用来统计或跟踪 java 的进程信息。

（1）、动态观察 GC 情况（阅读 GC日志发现频繁GC 、arthas观察、jvisualVM、Jprofiler等）。

[root@centos142 arthas]# jstat -gc 4869S0C    S1C    S0U    S1U      EC       EU        OC         OU       MC     MU    CCSC   CCSU   YGC     YGCT    FGC    FGCT     GCT   
8192.0 8192.0  0.0    0.0   50688.0  50040.7   136704.0   136606.0  4864.0 4113.3 512.0  446.4       5    0.304  12      5.061    5.366

（2）、每500毫秒刷新一次，用来跟踪内存的增长过程

[root@centos142 arthas]# jstat -gc 4869 500S0C    S1C    S0U    S1U      EC       EU        OC         OU       MC     MU    CCSC   CCSU   YGC     YGCT    FGC    FGCT     GCT   
8192.0 8192.0  0.0    0.0   50688.0  50551.2   136704.0   136606.0  4864.0 4113.3 512.0  446.4       5    0.304  21      8.917    9.221
8192.0 8192.0  0.0    0.0   50688.0  50600.3   136704.0   136606.0  4864.0 4113.3 512.0  446.4       5    0.304  21      8.917    9.221
8192.0 8192.0  0.0    0.0   50688.0  50653.5   136704.0   136606.0  4864.0 4113.3 512.0  446.4       5    0.304  21      8.917    9.221
8192.0 8192.0  0.0    0.0   50688.0  50653.5   136704.0   136606.0  4864.0 4113.3 512.0  446.4       5    0.304  21      8.917    9.221
8192.0 8192.0  0.0    0.0   50688.0  50653.5   136704.0   136606.0  4864.0 4113.3 512.0  446.4       5    0.304  21      8.917    9.221
8192.0 8192.0  0.0    0.0   50688.0  50653.5   136704.0   136606.0  4864.0 4113.3 512.0  446.4       5    0.304  21      8.917    9.221
8192.0 8192.0  0.0    0.0   50688.0  50655.6   136704.0   136606.0  4864.0 4113.3 512.0  446.4       5    0.304  21      8.917    9.221
8192.0 8192.0  0.0    0.0   50688.0  50688.0   136704.0   136606.0  4864.0 4113.3 512.0  446.4       5    0.304  22      8.917    9.221
8192.0 8192.0  0.0    0.0   50688.0  50039.5   136704.0   136606.0  4864.0 4113.3 512.0  446.4       5    0.304  22      9.302    9.606

注意：图形界面用在测试的时候， 压测观察，而不是用在生产环境下定位OOM。

4. jstack 4869 ：该命令会显示 所有的线程（线程名、线程编号、线程优先级、操作系统级的优先级、线程的状态、线程的调用堆栈）的信息。如：多个线程的状态一直是 waiting on condition  ，则说明程序可能产生了死锁。

如何找到哪个线程持有这把锁？

       搜索 jstack dump 的信息，找，看哪个线程持有这把锁 RUNNABLE。

[root@centos142 arthas]# jstack 4869
"pool-1-thread-45" #52 prio=5 os_prio=0 tid=0x00007fda34290000 nid=0x133d waiting on condition [0x00007fd9e2bea000]java.lang.Thread.State: WAITING (parking)at sun.misc.Unsafe.park(Native Method)- parking to wait for  <0x00000000f82aef40> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1088)at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1074)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)at java.lang.Thread.run(Thread.java:750)
"pool-1-thread-44" #51 prio=5 os_prio=0 tid=0x00007fda3428e000 nid=0x133c waiting on condition [0x00007fd9e2ceb000]java.lang.Thread.State: WAITING (parking)at sun.misc.Unsafe.park(Native Method)- parking to wait for  <0x00000000f82c4af8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870)at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199)

5. 系统级命令，top、top - Hp 进程号，top：查看当前进程占内存情况、top - Hp 进程号：查看进程号中所有线程的内存、cup情况。

6. jmap 作用1：

        a、jmap -histo 进程号： 对当前 jvm 的所有对象进行分析，查看哪些类有多少个对象，占多大字节。

        b、jmap -histo 进程号 | head -20 ：查看最占内存的前20个对象是哪些。

 jmap 作用2：

        jmap -dump:format=b,file=20230121.hprof 进程号  ：产生一个java的整个堆内存的转储文件，对它进行分析。

注意：jmap是不可以在 生成环境下执行的，会产生一个很严重的后果：jmap会让 jvm 卡死在某个状态，线上系统，内存特别大，jmap执行期间会对进程产生很大影响，甚至卡顿(电商不适合)

        1：设定了参数HeapDump，OOM的时候会自动产生堆转储文件(不是很专业，因为多有监控，内存增长就会报警)

        2：很多服务器备份(高可用) ，停掉这台服务器对其他服务器不影响

        3：在线定位(一般小点儿公司用不到)
        4：在测试环境中压测(产生类似内存增长问题，在堆还不是很大的时候进行转储)

JVM常见排查流程：

1. 使用 top 找出哪个进程占用 cpu 比较高。

2. 使用 top -Hp 进程号，找到这个进程中哪个线程占用 cpu 高。

3. 根据线程编号通过 jstack 查看调用链路。一般分为俩种情况：

        a、vm GC

        b、业务线程，找业务方法占用高的

4. 如果是 vm GC，查看日志，看看原因是什么。如：访问量压力大（扩容机器）、内存回收不掉。

5. 一般启动项目的时候会设置，当进程出现 OOM后，自动生成dump文件。通过工具对dump文件进行分析。

java -Xms200M -Xmx200M -XX:+UseParallelGC -XX:+HeapDumpOnOutOfMemoryError java文件或 jar包

以上所有命令，都可以被 arthas 替代，arthas更方便、更全面！，学习arthas地址：

Arthas 入门到实战（一）快速入门_明湖起风了的博客-CSDN博客