ASM 字节码替换功能未完成

Question

我有一个稍微复杂的架构，我试图在其中使用 JVMTI 函数 RetransformClasses, and event hook ClassLoadFileHook, while using JNI to go up to Java space to perform the transformation. In ClassLoadFileHook, I check if it's being fired for the class I want to instrument, and if so, I make a JNI call to a Java class where I perform the instrumentation using ASM 5.0.4 检测 class。也就是说，在 Java space 中，我得到一个表示已检测 class 的字节数组，然后回调到我的本机代码以应用更新。

我遇到的问题是，在我的 Java 代码中，我重新转换 class，代码挂起 during/after 我实例化了 ClassReader：

/**
 * Transform some class and then call back down to native code to apply
 * the transformation.
 *
 * @param existingClass The existing class to change.
 */
public synchronized void transformSomeClass(byte[] existingClass) {
    System.out.println("Transformation function started. Existing class len: " + existingClass.length);
    try {
        ClassReader cr = new ClassReader(existingClass);
        // No statements below here are being executed.
        // I'm also not sure if `new ClassReader` is being successfully
        // executed.
        System.out.println("After ClassReader instantiation");
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
        System.out.println("After ClassWriter instantiation");
        ClassVisitor cv =
                new MyCustomClassAdapter(cw, this.lineNumberToPlaceUpdate_);
        System.out.println("After ClassVisitor instantiation");
        cr.accept(cv, 0);
        System.out.println("After cv accept.");

        System.out.println("Applying class transformation natively.");
        // Call back down to native code to apply the update in ClassLoadFileHook.
        applyClassTransformNative(cw.toByteArray());
        System.out.println("Transformation function ended.");
    } catch (Exception e) {
        // This is never being fired.
        System.err.println("Class transformation could not be completed. Error message: " + e.getMessage());
        e.printStackTrace();
    } finally {
        // This print statement is never executed.
        System.err.println("Going back to native code from finally");
    }
}

正如我在代码注释中提到的，在 ClassReader 实例化语句之后什么都没有执行。即使 finally 块也不会执行。我不确定这里会发生什么——为什么实例化一个采用通用字节数组的 ClassReader 会导致代码像这样挂起？我实际上没有对 class.

执行任何操作

Answer 1

如果您调用 Java 代码导致 class 加载，则您在加载 class 的 class 加载器上隐式同步。我假设你在原生层触发了这样一个 class 加载，然后当你试图在 Java 层再次触发它时被阻止。这就是为什么当您尝试转换 Java 中的 class 时，您永远不会超越第一个 ASM class。

ASM 字节码替换功能未完成

ASM Bytecode replacement function not completing

java

java-native-interface

bytecode

java-bytecode-asm

javaagents