疑难问题：JDK 1.8 java.lang.IllegalStateException信息排查

背景：生产环境有一个批量创建问文件夹的接口，接口逻辑一次传入多个文件夹名称，去发号器批量申请文件夹id

某个场景下前端入参的文件夹名称重复，导致服务端报错如下

代码：

public class MyTest {
    public static void main(String[] args) {
        List<mydto> mydtoList= new ArrayList<>(2);
        mydto a = new mydto();
        a.setId(100L);
        a.setName("a");

        mydto b = new mydto();
        b.setId(101L);
        b.setName("a");
        mydtoList.add(a);
        mydtoList.add(b);

        Map<String,Long> d =mydtoList.stream().collect(Collectors.toMap(mydto::getName,mydto::getId));
    }

}

class mydto {

    private String name;

    private Long id;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

}

Exception in thread "main" java.lang.IllegalStateException: Duplicate key 100
	at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
	at java.util.HashMap.merge(HashMap.java:1256)
	at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1384)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
	at com.fancv.MyTest.main(MyTest.java:21)

神奇的报错信息

jdk 1.8 的报错信息有误导性性， Duplicate key 100，让人容易理解 id重复了，事实上是 name 重复了，可能是幸运女神眷顾，在整个业务场景下爆出这错，团队成员都误认为是id重复了，耽误了半天时间，后边证实是 name重复了

相比之下 jdk 11 的提示信息友好很多

Exception in thread "main" java.lang.IllegalStateException: Duplicate key a (attempted merging values 102 and 101)
	at java.base/java.util.stream.Collectors.duplicateKeyException(Collectors.java:133)
	at java.base/java.util.stream.Collectors.lambda$uniqKeysMapAccumulator$1(Collectors.java:180)
	at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1655)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
	at com.fancv.MyTest.main(MyTest.java:21)

排查问题的思考

jdk1.8 的报错信息已经定位到具体行数，根据上下文可以推理出不可能id重复，如果是其他业务场景和id没关系，追到这也就可以了
排查问题尽可能的还原现场，排除干扰， 有自己的判断，当然基础是自己技术扎实。

有条件的团队尽快升级jdk11，时间拖得越久，升级的成本越大。