JVM-问题定位示例

内存溢出定位与分析

1. 模拟内存溢出，向集合中添加极大数量的字符串

package com.example.testcpu;

import java.util.ArrayList;
import java.util.List;
import java.util.UUID;

public class TestJvmOutOfMemory {

    public static void main(String[] args) {
        List<Object> list = new ArrayList<>();
        for (int i = 0; i < 10000000; i++) {
            String str = "";
            for (int j = 0; j < 1000; j++) {
                str += UUID.randomUUID().toString();
            }
            list.add(str);
        }
        System.out.println("ok");
    }
}

2. 运行参数：

-Xms8m -Xmx8m -XX:+HeapDumpOnOutOfMemoryError

结果：

objc[78198]: Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/bin/java (0x103bf64c0) and /Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/libinstrument.dylib (0x103c7a4e0). One of the two will be used. Which one is undefined.
java.lang.OutOfMemoryError: Java heap space
Dumping heap to java_pid78198.hprof ...
Heap dump file created [7918632 bytes in 0.055 secs]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at java.util.Arrays.copyOf(Arrays.java:3332)
    at java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:137)
    at java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:121)
    at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:421)
    at java.lang.StringBuilder.append(StringBuilder.java:136)
    at com.example.testcpu.TestJvmOutOfMemory.main(TestJvmOutOfMemory.java:14)

Process finished with exit code 1

定位死锁问题

将dump文件导入到MAT中分析

发现服务器的CPU的负载突然增

高了、出现了死锁、死循环等，我们该如何分析呢？

1. 找到CPU占比高的进程

2. jstack查看线程情况，如发生死锁，会在最下面显示