MAT simple analysis of memory overflow problem

Simulate a memory overflow situation

import java.util.ArrayList;
import java.util.List;
import java.util.UUID;
/**
 * 向List集合中添加100万个字符串，每个字符串由1000个UUID组成。如果程  序能够正常执行，最后打印ok。
 */
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");
    }
}

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-Xms8m -Xmx8m -XX:+HeapDumpOnOutOfMemoryError

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Analysis with MAT

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It can be seen from the analysis results that 92.16% of the memory is occupied by Object[], so it is suspicious.

An overview of the main functions of the MAT interface

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Generate heap dump in real time

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Histogram

The bar chart shows the usage of each class, such as the number of classes, the space occupied, etc.
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grouping

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sort

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Regular search

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View citations

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code

import java.util.ArrayList;
import java.util.Random;
/**
 * -Xms30m -Xmx30m -XX:SurvivorRatio=8 -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps
   -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/opt/dump.hprof
 */
public class OOMTest {
    
    
    public static void main(String[] args) throws InterruptedException {
    
    
        ArrayList<Box> list = new ArrayList<>();
        while(true){
    
    
            Thread.sleep(100);
            list.add(new Box(new Random().nextInt(1024 * 1024)));
        }
    }
}

class Box{
    
    
    private byte[] data;

    public Box(int length) {
    
    
        this.data = new byte[length];
    }
}