There is no main thread in the SpringBoot web project - spring-boot

Question

After I start a SpringBoot web project. I can't find the main thread using jcmd $pid Thread.print. I also can't find the main thread using hsdb. Where did the main thread go?

Answer 1

I don't know the tool that you used in order to create your spring boot project, but if you created it via the Spring Initializr (https://start.spring.io/) it should be on the path YOUR_PROJECT_NAME/src/main/java/YOUR_PERSONALIZED_PATH/.
The name of the file where the main thread is created/executed should be inside that path, and should be called YOUR_PROJECT_NAME+Application.java.

Answer 2

For most Spring Boot apps, SpringApplication::run involves starting a web server (Tomcat, Undertow, Jetty, Netty). Those servers create their own non-daemon threads. The call to SpringApplication::run then returns and the main thread exits. The VM then is kept alive by those other non-daemon threads – the exact names depend on the web server used.
#SpringBootApplication
public class App {
public static void main(String[] args) {
SpringApplication.run(App.class, args);
}
}

Answer 3

The web application delegates the main method to SpringApplication.run, which will execute Spring initialization and other processes. After the Spring initialization is completed, the life cycle of the main method has ended.
Why doesn't the java process exit without the main method?
java.c
/* Build platform specific argument array */
mainArgs = CreateApplicationArgs(env, argv, argc);
CHECK_EXCEPTION_NULL_LEAVE(mainArgs);
/* Invoke main method. */
(*env)->CallStaticVoidMethod(env, mainClass, mainID, mainArgs);
/*
* The launcher's exit code (in the absence of calls to
* System.exit) will be non-zero if main threw an exception.
*/
ret = (*env)->ExceptionOccurred(env) == NULL ? 0 : 1;
LEAVE();
#define LEAVE() \
do { \
if ((*vm)->DetachCurrentThread(vm) != JNI_OK) { \
JLI_ReportErrorMessage(JVM_ERROR2); \
ret = 1; \
} \
if (JNI_TRUE) { \
(*vm)->DestroyJavaVM(vm); \
return ret; \
} \
} while (JNI_FALSE)
Because it is stated in the comments of the macro definition of leave.
Always detach the main thread so that it appears to have ended when the application's main method exits. This will invoke the uncaught exception handler machinery if main threw an exception. An uncaught exception handler cannot change the launcher's return code except by calling System.exit.
Wait for all non-daemon threads to end, then destroy the VM. This will actually create a trivial new Java waiter thread named "DestroyJavaVM", but this will be seen as a different thread from the one that executed main, even though they are the same C thread. This allows mainThread.join() an mainThread.isAlive() to work as expected.
In this case, process has non-daemon thread so the process not exit.