为什么 JVM 会为同一个 class 发送多个 JVMTI ClassLoad 事件？

Question

我启用了 JVMTI ClassLoad 事件，这些事件应该在首次加载 class 时生成。我希望每个 class 加载一次，但对于某些 classes，它似乎会生成多次，例如这个我得到了两次：

ClassLoad: Ljava/util/concurrent/ThreadFactory; loaded by thread 1
ClassLoad: Ljava/util/concurrent/ThreadFactory; loaded by thread 1

在 systemDictionary.cpp 中，我发现了三个 JvmtiExport::post_class_load，我认为这是负责执行回调（如果有）的代码。他们在

SystemDictionary::resolve_instance_class_or_null
SystemDictionary::parse_stream
SystemDictionary::define_instance_class

但我还不了解整个流程，因此不清楚为什么我会多次收到该事件。

我能做些什么来防止这种情况发生，还是我需要自己处理并调整代码以忽略同一事件的多个事件class？

Answer 1

活动看两遍就OK了。这意味着 class 解析在不同的 class 加载程序的上下文中发生两次。首先，它由 Bootstrap class 加载器解析，然后由系统 class 加载器解析 - sun.misc.Launcher.AppClassLoader.

来自的实用程序将阐明此类事件发生时的情况。

考虑一个简单的例子：

public class Test {

    public void setThreadFactory(ThreadFactory factory) {
    }

    public static void main(String[] args) {
    }
}

当启动器查找 public static void main() 时，它会为 public 方法创建 Method 对象，这会导致解析此类方法签名中的所有 classes .

第一个 ClassLoad 事件：

Class loaded: java/util/concurrent/ThreadFactory
  - ClassLoad(_jvmtiEnv*, JNIEnv_*, _jobject*, _jclass*) + 0x69
  - JvmtiExport::post_class_load(JavaThread*, Klass*) + 0x15b
  - SystemDictionary::define_instance_class(instanceKlassHandle, Thread*) + 0x3cc
  - SystemDictionary::find_or_define_instance_class(Symbol*, Handle, instanceKlassHandle, Thread*) + 0x35d
  - SystemDictionary::load_instance_class(Symbol*, Handle, Thread*) + 0x20c
  - SystemDictionary::resolve_instance_class_or_null(Symbol*, Handle, Handle, Thread*) + 0x78c
  - JVM_FindClassFromBootLoader + 0x22b
  - Java_java_lang_ClassLoader_findBootstrapClass + 0x9b
  * java/lang/ClassLoader.findBootstrapClass @ -1
  * java/lang/ClassLoader.findBootstrapClassOrNull @ 12
  * java/lang/ClassLoader.loadClass @ 48
  * java/lang/ClassLoader.loadClass @ 38
  * sun/misc/Launcher$AppClassLoader.loadClass @ 81
  * java/lang/ClassLoader.loadClass @ 3
  * java/lang/Class.getDeclaredMethods0 @ -1
  * java/lang/Class.privateGetDeclaredMethods @ 37
  * java/lang/Class.privateGetMethodRecursive @ 2
  * java/lang/Class.getMethod0 @ 16
  * java/lang/Class.getMethod @ 13
  * sun/launcher/LauncherHelper.validateMainClass @ 12
  * sun/launcher/LauncherHelper.checkAndLoadMain @ 214

第二个：

Class loaded: java/util/concurrent/ThreadFactory
  - ClassLoad(_jvmtiEnv*, JNIEnv_*, _jobject*, _jclass*) + 0x69
  - JvmtiExport::post_class_load(JavaThread*, Klass*) + 0x15b
  - SystemDictionary::resolve_instance_class_or_null(Symbol*, Handle, Handle, Thread*) + 0x87c
  - SystemDictionary::resolve_or_fail(Symbol*, Handle, Handle, bool, Thread*) + 0x33
  - get_mirror_from_signature(methodHandle, SignatureStream*, Thread*) + 0xc6
  - Reflection::get_parameter_types(methodHandle, int, oopDesc**, Thread*) + 0x18e
  - Reflection::new_method(methodHandle, bool, bool, Thread*) + 0xfc
  - get_class_declared_methods_helper(JNIEnv_*, _jclass*, unsigned char, bool, Klass*, Thread*) + 0x479
  - JVM_GetClassDeclaredMethods + 0xcb
  * java/lang/Class.getDeclaredMethods0 @ -1
  * java/lang/Class.privateGetDeclaredMethods @ 37
  * java/lang/Class.privateGetMethodRecursive @ 2
  * java/lang/Class.getMethod0 @ 16
  * java/lang/Class.getMethod @ 13
  * sun/launcher/LauncherHelper.validateMainClass @ 12
  * sun/launcher/LauncherHelper.checkAndLoadMain @ 214

为什么 JVM 会为同一个 class 发送多个 JVMTI ClassLoad 事件？

Why does the JVM send multiple JVMTI ClassLoad events for the same class?

java

jvm

classloader

jvmti