OpenJDK HotSpot JVM：如何定位main方法类及主线程识别逻辑？

Hey there! Let's break down your questions about locating and logging the main class in OpenJDK HotSpot's source code step by step:

1. How to find where the main() class is handled in HotSpot source? #

The HotSpot JVM doesn't "search" for the main class on its own—it gets the class name from the launcher that starts the JVM. Here's the key flow:

• First, the native launcher (look at src/java.base/share/native/launcher/main.c) parses command-line arguments. If you run a class directly, it grabs the class name from the args; if you run a JAR, it reads the Main-Class attribute from the JAR's manifest.
• This class name is passed to the JVM during initialization. Inside HotSpot, the Arguments class (defined in src/hotspot/share/runtime/arguments.cpp) stores this value. You can access it via the Arguments::main_class() method, which returns a Symbol* representing the fully qualified main class name (like com.example.MyApp).

2. Logging the main class name/package early (right around VM creation) #

Since you've already found Threads::create_vm, you're in the right spot to add your logging. Here's how to do it:

Step 1: Access the main class name #

The Arguments::main_class() method gives you the fully qualified name of the main class. To print it as a C string, use symbol->as_C_string().

Step 2: Add logging code at the right place #

You have two good options for early logging:

• After VM initialization (inside Threads::create_vm): At the end of the Threads::create_vm function in src/hotspot/share/runtime/thread.cpp, right before the method returns success, add:

  if (Arguments::main_class() != nullptr) {
    const char* main_class_str = Arguments::main_class()->as_C_string();
    printf("Detected main class: %s\n", main_class_str);
    
    // Split into package and class name
    const char* last_dot = strrchr(main_class_str, '.');
    if (last_dot != nullptr) {
      printf("Package name: %.*s\n", (int)(last_dot - main_class_str), main_class_str);
      printf("Simple class name: %s\n", last_dot + 1);
    } else {
      printf("Package name: (no package)\n");
      printf("Simple class name: %s\n", main_class_str);
    }
  }
  

• Even earlier (after argument parsing): If you want to log before VM initialization completes, add the same code at the end of Arguments::parse() in src/hotspot/share/runtime/arguments.cpp. This runs right after the launcher passes the main class name to the JVM.

Notes #

• Make sure to include <cstring> for strrchr if it's not already included in the file.
• This works for both direct class execution and JAR files, since Arguments::main_class() handles both cases correctly.

内容的提问来源于stack exchange，提问作者Markus Weninger