字节流生成的map进行remove报错分析

使用stream流生成的map进行remove操作会报错

当对stream生成的map进行remove操作时，代码报错，复现代码如下：

package com.test.testdemo01.service;

import com.test.testdemo01.entity.dto.DemoData;
import org.junit.Test;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

/**
 * @author : xubin
 * @date : 2023/12/14
 * @description : 针对字节流生成的map存在移除问题，针对异常情况进行测试及提供解决方案
 */
public class IterationDemo {



    /**
     * 测试使用字节流会造成报错
     */
    @Test
    public void test1(){
        //数据准备
        List demoDataList = new ArrayList<>();
        DemoData demoData1 = new DemoData();
        demoData1.setName("zhangsan");
        demoData1.setSno(123);
        DemoData demoData2 = new DemoData();
        demoData2.setName("zhangsan");
        demoData2.setSno(456);
        DemoData demoData3= new DemoData();
        demoData3.setName("lisi");
        demoData3.setSno(123);
        demoDataList.add(demoData1);
        demoDataList.add(demoData2);
        demoDataList.add(demoData3);


        Map> collect = demoDataList.stream().
                            collect(Collectors.groupingBy(DemoData::getName));

        for (String key : collect.keySet()){
            if ("lisi".equals(key)){
                collect.remove(key);
            }
        }


    }

}

DemoData实体：

package com.test.testdemo01.entity.dto;

import com.alibaba.excel.annotation.ExcelProperty;
import lombok.Data;

import java.util.UUID;

/**
 * @author : xubin
 * @date : 2022/11/21
 * @description :
 */
@Data
public class DemoData {
    //设置excel表头名称
    @ExcelProperty("学生编号")
    private Integer sno;

    @ExcelProperty("学生的姓名")
    private String name;

    public static void main(String[] args) {
        System.out.println(UUID.randomUUID()
        );
    }
}

报错详情:

java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)
    at java.util.HashMap$KeyIterator.next(HashMap.java:1469)
    at com.test.testdemo01.service.IterationDemo.test1(IterationDemo.java:43)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
    at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
    at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
    at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
    at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
    at com.intellij.rt.junit.IdeaTestRunner$Repeater$1.execute(IdeaTestRunner.java:38)
    at com.intellij.rt.execution.junit.TestsRepeater.repeat(TestsRepeater.java:11)
    at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:35)
    at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:232)
    at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:55)


Process finished with exit code -1

使用新建的hashmap没有这个问题的发生

代码：

package com.test.testdemo01.service;

import com.alibaba.fastjson.JSON;
import com.test.testdemo01.entity.dto.DemoData;
import com.test.testdemo01.entity.dto.TbOrderSendFilePO;
import org.apache.commons.collections4.map.HashedMap;
import org.junit.Test;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

/**
 * @author : xubin
 * @date : 2023/12/14
 * @description : 针对字节流生成的map存在移除问题，针对异常情况进行测试及提供解决方案
 */
public class IterationDemo {

    /**
     * 使用普通hashmap没有这个问题
     */
    @Test
    public void test3(){
        //数据准备
        List demoDataList = new ArrayList<>();
        DemoData demoData1 = new DemoData();
        demoData1.setName("zhangsan");
        demoData1.setSno(123);
        DemoData demoData2 = new DemoData();
        demoData2.setName("zhangsan");
        demoData2.setSno(456);
        DemoData demoData3= new DemoData();
        demoData3.setName("lisi");
        demoData3.setSno(123);
        demoDataList.add(demoData1);
        demoDataList.add(demoData2);
        demoDataList.add(demoData3);


        Map> collect = new HashedMap<>();
        List demoDataList1 = new ArrayList<>();
        demoDataList1.add(demoData1);
        demoDataList1.add(demoData2);
        List demoDataList2 = new ArrayList<>();
        demoDataList2.add(demoData3);
        collect.put("zhangsan",demoDataList1);
        collect.put("lisi",demoDataList2);
        System.out.println(JSON.toJSONString(collect));

        for (String key : collect.keySet()){
            if ("lisi".equals(key)){
                System.out.println("need delete");
                collect.remove(key);
            }
        }
        System.out.println(JSON.toJSONString(collect));


    }



}

输出结果：

{"zhangsan":[{"name":"zhangsan","sno":123},{"name":"zhangsan","sno":456}],"lisi":[{"name":"lisi","sno":123}]}
need delete
{"zhangsan":[{"name":"zhangsan","sno":123},{"name":"zhangsan","sno":456}]}

Process finished with exit code 0

使用迭代器避免这个问题的发生

修改后代码

package com.test.testdemo01.service;

import com.alibaba.fastjson.JSON;
import com.test.testdemo01.entity.dto.DemoData;
import com.test.testdemo01.entity.dto.TbOrderSendFilePO;
import org.junit.Test;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

/**
 * @author : xubin
 * @date : 2023/12/14
 * @description : 针对字节流生成的map存在移除问题，针对异常情况进行测试及提供解决方案
 */
public class IterationDemo {


    /**
     * 使用迭代器能够避免这个问题
     */
    @Test
    public void test2(){
        //数据准备
        List demoDataList = new ArrayList<>();
        DemoData demoData1 = new DemoData();
        demoData1.setName("zhangsan");
        demoData1.setSno(123);
        DemoData demoData2 = new DemoData();
        demoData2.setName("zhangsan");
        demoData2.setSno(456);
        DemoData demoData3= new DemoData();
        demoData3.setName("lisi");
        demoData3.setSno(123);
        demoDataList.add(demoData1);
        demoDataList.add(demoData2);
        demoDataList.add(demoData3);


        Map> collect = demoDataList.stream().
                        collect(Collectors.groupingBy(DemoData::getName));

        System.out.println(JSON.toJSONString(collect));
        Iterator>> iterator1 = 
                        collect.entrySet().iterator();
        while (iterator1.hasNext()){
            Map.Entry> next = iterator1.next();
            if ("lisi".equals(next.getKey())){
                System.out.println("need delete");
                iterator1.remove();
            }

        }
        System.out.println(JSON.toJSONString(collect));


    }

}

输出日志：

{"lisi":[{"name":"lisi","sno":123}],"zhangsan":[{"name":"zhangsan","sno":123},{"name":"zhangsan","sno":456}]}
need delete
{"zhangsan":[{"name":"zhangsan","sno":123},{"name":"zhangsan","sno":456}]}

Process finished with exit code 0