Interviewer: If you encounter OOM online, how to solve it?

OOM can be said to be one of the most feared problems for our developers. The causes are basically caused by code or JVM parameter configuration.

This article talks to readers about how to troubleshoot after the Java process triggers OOM.

It is often said to be in awe of the production environment, and solving problems quickly is also a sign of awe

Why Will OOM

OOM stands for "Out Of Memory", which means the memory is exhausted. When the JVM does not have enough memory to allocate space for the object, and the garbage collector has no space to recycle, it will throw this error

Why does OOM occur? It is generally caused by these problems

1. Too little allocation: The JVM initialization memory is small and the business uses a lot of memory; or the memory allocation in different JVM areas is unreasonable
2. Code vulnerability: a certain object Frequently applied for but not released after no use, resulting in memory exhaustion

Memory leak: The memory used after application is not released, causing the virtual machine to be unable to be used again Using this memory, this memory is leaked. Because the applicant is no longer in use and cannot be allocated to others by the virtual machine

Memory overflow: The requested memory exceeds the memory size that the JVM can provide. This is called an overflow

Memory leaks persist and will eventually overflow. The two are causally related

Common OOM

The more common OOM types include the following

java.lang.OutOfMemoryError: PermGen space

Java7 permanent generation (method area) overflow, which is used Stores data such as class information, constants, static variables, and code compiled by the just-in-time compiler that have been loaded by the virtual machine. Whenever a class is loaded for the first time, metadata will be stored in the permanent generation

Generally appears in a large number of Class objects or JSP pages, or the use of CgLib dynamic proxy technology causes

We can pass -XX: PermSize and -XX: MaxPermSize Modify the size of the method area

Java8 changes the permanent generation to the metaspace, an error is reported: java.lang.OutOfMemoryError: Metadata space, the metaspace memory is insufficient and dynamically expanded by default

java.lang.StackOverflowError

Virtual machine stack overflow is usually caused by the existence of an infinite loop or deep recursive call in the program. If the stack size is set too small, overflow will occur. You can set the stack size through -Xss

The virtual machine throws a stack overflow error, and you can locate the wrong class and method in the log.

java.lang.OutOfMemoryError: Java heap space

Java heap memory overflow, the cause of overflow is generally due to unreasonable JVM heap memory settings or Memory leaks lead to

If there is a memory leak, you can use tools to view the reference chain from the leaked object to GC Roots. By mastering the type information of the leaked object and the GC Roots reference chain information, you can accurately locate the location of the leaked code

If there is no memory leak, that is, the objects in the memory must still be alive, then You should check the heap parameters of the virtual machine (-Xmx and -Xms) to see if the memory of the virtual machine can be increased.

Summary: The overflow scenario of the method area and the virtual machine stack will not be discussed too much in this article. The following mainly explains the common OOM troubleshooting ideas of Java heap space

Check the JVM memory distribution

Assume that our Java application The PID is 15162. Enter the command to view the JVM memory distribution jmap -heap 15162

[xxx@xxx ~]# jmap -heap 15162
Attaching to process ID 15162, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.161-b12

using thread-local object allocation.
Mark Sweep Compact GC

Heap Configuration:
   MinHeapFreeRatio         = 40 # 最小堆使用比例
   MaxHeapFreeRatio         = 70 # 最大堆可用比例
   MaxHeapSize              = 482344960 (460.0MB) # 最大堆空间大小
   NewSize                  = 10485760 (10.0MB) # 新生代分配大小
   MaxNewSize               = 160759808 (153.3125MB) # 最大新生代可分配大小
   OldSize                  = 20971520 (20.0MB) # 老年代大小
   NewRatio                 = 2 # 新生代比例
   SurvivorRatio            = 8 # 新生代与 Survivor 比例
   MetaspaceSize            = 21807104 (20.796875MB) # 元空间大小
   CompressedClassSpaceSize = 1073741824 (1024.0MB) # Compressed Class Space 空间大小限制
   MaxMetaspaceSize         = 17592186044415 MB # 最大元空间大小
   G1HeapRegionSize         = 0 (0.0MB) # G1 单个 Region 大小

Heap Usage:  # 堆使用情况
New Generation (Eden + 1 Survivor Space): # 新生代
   capacity = 9502720 (9.0625MB) # 新生代总容量
   used     = 4995320 (4.763908386230469MB) # 新生代已使用
   free     = 4507400 (4.298591613769531MB) # 新生代剩余容量
   52.56726495150862% used # 新生代使用占比
Eden Space:
   capacity = 8454144 (8.0625MB) # Eden 区总容量
   used     = 4029752 (3.8430709838867188MB) # Eden 区已使用
   free     = 4424392 (4.219429016113281MB) # Eden 区剩余容量
   47.665996699370154% used  # Eden 区使用占比
From Space: # 其中一个 Survivor 区的内存分布
   capacity = 1048576 (1.0MB)
   used     = 965568 (0.92083740234375MB)
   free     = 83008 (0.07916259765625MB)
   92.083740234375% used
To Space: # 另一个 Survivor 区的内存分布
   capacity = 1048576 (1.0MB)
   used     = 0 (0.0MB)
   free     = 1048576 (1.0MB)
   0.0% used
tenured generation: # 老年代
   capacity = 20971520 (20.0MB)
   used     = 10611384 (10.119804382324219MB)
   free     = 10360136 (9.880195617675781MB)
   50.599021911621094% used

10730 interned Strings occupying 906232 bytes.

By viewing the JVM memory allocation and runtime usage, you can determine whether the memory allocation is reasonable

In addition, you can view the most resource-consuming objects while the JVM is running, jmap -histo:live 15162 | more

The JVM memory object list is sorted according to the memory size occupied by the object

• instances: number of instances
• bytes: unit byte
• class name: class name

It is obvious that CustomObjTest object instances and excessive memory usage

Unfortunately, the solution has limitations. Because it can only troubleshoot the problem of excessive memory usage by objects

where "[" represents an array, for example, "[C" represents a Char array, and "[B" represents a Byte array. If the array memory takes up too much, we don’t know which objects hold it, so we need to Dump the memory for offline analysis

jmap -histo:live Execute this command, the JVM will Trigger GC first, then collect statistics

Dump file analysis

The Dump file is the memory of the Java process Mirror, which mainly includes System information, Virtual machine properties, Complete thread Dump, Status of all classes and objects and other information

When a memory overflow or GC exception occurs in the program, it is suspected that the JVM has memory leak, then we can export the Dump file for analysis

Add the following to the JVM startup parameter configuration parameter

• -XX:+HeapDumpOnOutOfMemoryError
• -XX:HeapDumpPath=./（参数为 Dump 文件生成路径）

当 JVM 发生 OOM 异常自动导出 Dump 文件，文件名称默认格式：java_pid{pid}.hprof

上面配置是在应用抛出 OOM 后自动导出 Dump，或者可以在 JVM 运行时导出 Dump 文件

jmap -dump:file=[文件路径] [pid]

# 示例
jmap -dump:file=./jvmdump.hprof 15162

在本地写一个测试代码，验证下 OOM 以及分析 Dump 文件

设置 VM 参数：-Xms3m -Xmx3m -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=./

public static void main(String[] args) {
    List<Object> oomList = Lists.newArrayList();
   // 无限循环创建对象
    while (true) {
        oomList.add(new Object());
    }
}

通过报错信息得知，java heap space 表示 OOM 发生在堆区，并生成了 hprof 二进制文件在当前文件夹下

JvisualVM 分析

Dump 分析工具有很多，相对而言 JvisualVM、JProfiler、Eclipse Mat，使用人群更多一些。下面以 JvisualVM 举例分析 Dump 文件

列举两个常用的功能，第一个是能看到触发 OOM 的线程堆栈，清晰得知程序溢出的原因

第二个就是可以查看 JVM 内存里保留大小最大的对象，可以自由选择排查个数

点击对象还可以跳转具体的对象引用详情页面

文中 Dump 文件较为简单，而正式环境出错的原因五花八门，所以不对该 Dump 文件做深度解析

注意：JvisualVM 如果分析大 Dump 文件，可能会因为内存不足打不开，需要调整默认的内存

Summary review

If you encounter JVM memory overflow online, you can troubleshoot in the following steps

1. jmap -heap Check whether the memory allocation is too small
2. ##jmap -histo Check whether there is any obvious object allocation There are many and no releases
3. jmap -dump Export the JVM current memory snapshot, and use tools such as JDK or MAT to analyze the snapshot

If the problem cannot be located above, you need to check whether the application is constantly creating resources, such as network connections or threads, which may cause system resources to be exhausted.

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