[Bug] When the thread context classloader is null the InstanceMethodInterTemplate class will throw NullPointerException #13461

sweetcczhang · 2025-09-04T06:47:18Z

sweetcczhang
Sep 4, 2025

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I had searched in the issues and found no similar issues.

Apache SkyWalking Component

Java Agent (apache/skywalking-java)

What happened

when I add jdk-forkjoinpool plugin in my skywalking-java agent and there are code in my application like below:

    @RequestMapping("/test2")
    public String test2() {
        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < 6; i++) {
            list.add(i);
        }
        list.parallelStream().forEach(integer -> {
            System.out.println("------- print log:" + integer);
        });

        return "success";
    }

Accessing this endpoint via HTTP requests will cause the requests to hang. and the application log is below:

04-Sep-2025 11:24:46.999 信息 [Catalina-utility-1] org.apache.catalina.startup.HostConfig.deployDirectory 把web 应用程序部署到目录 [/software/apache-tomcat-9.0.108/webapps/manager]
------- print log:3
Exception in thread "ForkJoinPool.commonPool-worker-9" Exception in thread "ForkJoinPool.commonPool-worker-2" java.lang.NullPointerException
	at org.apache.skywalking.apm.plugin.jdk.forkjoinpool.ForkJoinWorkerQueueMethodInterceptor_internal.prepare(InstanceMethodInterTemplate.java:131)
	at org.apache.skywalking.apm.plugin.jdk.forkjoinpool.ForkJoinWorkerQueueMethodInterceptor_internal.intercept(InstanceMethodInterTemplate.java:70)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1703)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:172)
java.lang.NullPointerException
	at org.apache.skywalking.apm.plugin.jdk.forkjoinpool.ForkJoinWorkerQueueMethodInterceptor_internal.prepare(InstanceMethodInterTemplate.java:131)
	at org.apache.skywalking.apm.plugin.jdk.forkjoinpool.ForkJoinWorkerQueueMethodInterceptor_internal.intercept(InstanceMethodInterTemplate.java:70)

As indicated by the logs, a NullPointerException was thrown at line 131 of the InstanceMethodInterTemplate class. The relevant code is shown below

private static void prepare() {
        if (INTERCEPTOR == null) {
            ClassLoader loader = BootstrapInterRuntimeAssist.getAgentClassLoader();

            if (loader != null) {
                IBootstrapLog logger = BootstrapInterRuntimeAssist.getLogger(loader, TARGET_INTERCEPTOR);
                if (logger != null) {
                    LOGGER = logger;

                    INTERCEPTOR = BootstrapInterRuntimeAssist.createInterceptor(loader, TARGET_INTERCEPTOR, LOGGER);
                }
            } else {
                LOGGER.error("Runtime ClassLoader not found when create {}." + TARGET_INTERCEPTOR);
            }
        }
    }

As evidenced by the InstanceMethodInterTemplate code, when loader == null, the LOGGER is not initialized, thus invoking logging operations will throw a NullPointerException. And This exception will be propagated to the user program, disrupting the execution of business logic.

What you expected to happen

As indicated by the above description, this issue is caused by loader == null. Examining the code of the getAgentClassLoader method in the BootstrapInterRuntimeAssist class reveals that when the thread context ClassLoader is null, this situation occurs. the code is below:

public static ClassLoader getAgentClassLoader() {
        try {
            ClassLoader loader = Thread.currentThread().getContextClassLoader();
            if (loader == null) {
                return null;
            }
            Class<?> agentClassLoaderClass = Class.forName(AGENT_CLASSLOADER_DEFAULT, true, loader);
            Field defaultLoaderField = agentClassLoaderClass.getDeclaredField(DEFAULT_AGENT_CLASSLOADER_INSTANCE);
            defaultLoaderField.setAccessible(true);
            ClassLoader defaultAgentClassLoader = (ClassLoader) defaultLoaderField.get(null);

            return defaultAgentClassLoader;
        } catch (Exception e) {
            e.printStackTrace(OUT);
            return null;
        }
    }

In the test case, without adding the SkyWalking agent and printing the thread context ClassLoader as follows:. The logs show that the ClassLoader of threads in the ForkJoin pool is null.
the test code is below:

@RequestMapping("/test2")
    public String test2() {
        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < 6; i++) {
            list.add(i);
        }
        list.parallelStream().forEach(integer -> {
            System.out.println("------- print log:" + integer);
            System.out.println("----thread=" + Thread.currentThread().getName() + "------- classloader:" + Thread.currentThread().getContextClassLoader());
        });

        return "success";
    }

and the log is below:

------- print log:1
----thread=ForkJoinPool.commonPool-worker-9------- classloader:null
------- print log:2
----thread=ForkJoinPool.commonPool-worker-9------- classloader:null
------- print log:5
----thread=ForkJoinPool.commonPool-worker-2------- classloader:null
------- print log:4
----thread=ForkJoinPool.commonPool-worker-13------- classloader:null
------- print log:0
----thread=ForkJoinPool.commonPool-worker-11------- classloader:null
------- print log:3
----thread=http-nio-8080-exec-3------- classloader:ParallelWebappClassLoader
  context: ROOT
  delegate: false
----------> Parent Classloader:
java.net.URLClassLoader@3fc7c734

How to reproduce

jdk：1.8.0_431
spring boot : 2.5.9
tomcat: 9.0.108
skywalking-java: latest version. And you need to put the jdk-forkjoinpool plugin jar file into /plugins.
test application code:

@RequestMapping("/test2")
    public String test2() {
        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < 6; i++) {
            list.add(i);
        }
        list.parallelStream().forEach(integer -> {
            System.out.println("------- print log:" + integer);
        });

        return "success";
    }

attention: This issue only occurs when the application is packaged as a WAR file and deployed to Tomcat, whereas it does not occur when directly using Spring Boot's embedded Tomcat.

Anything else

No response

Are you willing to submit a pull request to fix on your own?

Yes I am willing to submit a pull request on my own!

Code of Conduct

I agree to follow this project's Code of Conduct

wu-sheng · 2025-09-04T07:02:07Z

wu-sheng
Sep 4, 2025
Collaborator

We never threw an agent core internal error to the use codes. What is point of that?

3 replies

wu-sheng Sep 4, 2025
Collaborator

All agent/plugins should be tested before they are deployed into the product environment.

sweetcczhang Sep 4, 2025
Author

In certain scenarios, InstanceMethodInterTemplate may throw a NullPointerException. the InstanceMethodInterTemplate class code is below. For example, when loader == null, the code executes: LOGGER.error("Runtime ClassLoader not found when create {}." + TARGET_INTERCEPTOR);. However, since the LOGGER itself is uninitialized at this point, a NullPointerException will be thrown.

private static void prepare() {
        if (INTERCEPTOR == null) {
            ClassLoader loader = BootstrapInterRuntimeAssist.getAgentClassLoader();

            if (loader != null) {
                IBootstrapLog logger = BootstrapInterRuntimeAssist.getLogger(loader, TARGET_INTERCEPTOR);
                if (logger != null) {
                    LOGGER = logger;

                    INTERCEPTOR = BootstrapInterRuntimeAssist.createInterceptor(loader, TARGET_INTERCEPTOR, LOGGER);
                }
            } else {
                LOGGER.error("Runtime ClassLoader not found when create {}." + TARGET_INTERCEPTOR);
            }
        }
    }

wu-sheng Sep 4, 2025
Collaborator

Then, you should focus on why this Bootstrap plugin fails, actually, rather than NPE. We tested in many environments, this doesn't happen.

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[Bug] When the thread context classloader is null the InstanceMethodInterTemplate class will throw NullPointerException #13461

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sweetcczhang Sep 4, 2025

Search before asking

Apache SkyWalking Component

What happened

What you expected to happen

How to reproduce

Anything else

Are you willing to submit a pull request to fix on your own?

Code of Conduct

Replies: 1 comment · 3 replies

Uh oh!

wu-sheng Sep 4, 2025 Collaborator

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wu-sheng Sep 4, 2025 Collaborator

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sweetcczhang Sep 4, 2025 Author

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wu-sheng Sep 4, 2025 Collaborator

sweetcczhang
Sep 4, 2025

Replies: 1 comment 3 replies

wu-sheng
Sep 4, 2025
Collaborator

wu-sheng Sep 4, 2025
Collaborator

sweetcczhang Sep 4, 2025
Author

wu-sheng Sep 4, 2025
Collaborator