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

[sweetcczhang]

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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]

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

[wu-sheng]

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

[sweetcczhang]

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]

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