是否有可能追踪导致NPE的表达？

Question

当我得到一个NPE时,我会得到一个带行号的堆栈跟踪.这很有用,但是如果行非常密集和/或包含嵌套表达式,那么仍然无法确定哪个引用为null.

当然,这些信息一定在某处可用.有没有办法解决这个问题？(如果不是java表达式,那么至少导致NPE的字节码指令也会有帮助)

编辑#1:我看过一些评论暗示分手等等,没有任何冒犯,实际上是非建设性和无关紧要的.如果我能做到这一点,我会的!我只想说修改源是不可能的.

编辑#2:apangin在下面发布了一个很好的答案,我接受了.但是,对于那些不想自己尝试的人来说,我必须将输出包含在这里!;)

假设我有这个驱动程序TestNPE.java

 1  public class TestNPE {
 2      public static void main(String[] args) {
 3          int n = 0;
 4          String st = null;
 5  
 6          System.out.println("about to throw NPE");
 7          if (n >= 0 && st.isEmpty()){
 8              System.out.println("empty");
 9          }
10          else {
11              System.out.println("othereise");
12          }
13      }
14      
15  }

字节码看起来像这样(仅显示main()方法并省略其他不相关的部分)

Code:
  stack=2, locals=3, args_size=1
     0: iconst_0
     1: istore_1
     2: aconst_null
     3: astore_2
     4: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;                                              
     7: ldc           #3                  // String about to throw NPE                                                                     
     9: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V                                      
    12: iload_1
    13: iflt          34
    16: aload_2
    17: invokevirtual #5                  // Method java/lang/String.isEmpty:()Z                                                           
    20: ifeq          34
    23: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;                                              
    26: ldc           #6                  // String empty                                                                                  
    28: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V                                      
    31: goto          42
    34: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;                                              
    37: ldc           #7                  // String othereise                                                                              
    39: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V                                      
    42: return

现在,当您使用代理运行TestNPE驱动程序时,您将获得此信息

$ java -agentpath:libRichNPE.o TestNPE
about to throw NPE
Exception in thread "main" java.lang.NullPointerException: location=17
    at TestNPE.main(TestNPE.java:7)

所以指向偏移17处的invokevirtual#5!这是怎么回事？

Answer 1

发生异常时,JVM知道导致异常的原始字节码.但是,StackTraceElement不跟踪字节码索引.

解决方案是在发生异常时使用JVMTI捕获字节码索引.

以下示例JVMTI代理将拦截所有异常,如果异常类型为NullPointerException,则代理将detailMessage使用字节码位置信息替换它.

#include <jvmti.h>
#include <stdio.h>

static jclass NullPointerException;
static jfieldID detailMessage;

void JNICALL VMInit(jvmtiEnv* jvmti, JNIEnv* env, jthread thread) {
    jclass localNPE = env->FindClass("java/lang/NullPointerException");
    NullPointerException = (jclass) env->NewGlobalRef(localNPE);

    jclass Throwable = env->FindClass("java/lang/Throwable");
    detailMessage = env->GetFieldID(Throwable, "detailMessage", "Ljava/lang/String;");
}

void JNICALL ExceptionCallback(jvmtiEnv* jvmti, JNIEnv* env, jthread thread,
                               jmethodID method, jlocation location, jobject exception,
                               jmethodID catch_method, jlocation catch_location) {
    if (env->IsInstanceOf(exception, NullPointerException)) {
        char buf[32];
        sprintf(buf, "location=%ld", (long)location);
        env->SetObjectField(exception, detailMessage, env->NewStringUTF(buf));
    }
}

JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM* vm, char* options, void* reserved) {
    jvmtiEnv* jvmti;
    vm->GetEnv((void**)&jvmti, JVMTI_VERSION_1_0);

    jvmtiCapabilities capabilities = {0};
    capabilities.can_generate_exception_events = 1;
    jvmti->AddCapabilities(&capabilities);

    jvmtiEventCallbacks callbacks = {0};
    callbacks.VMInit = VMInit;
    callbacks.Exception = ExceptionCallback;
    jvmti->SetEventCallbacks(&callbacks, sizeof(callbacks));
    jvmti->SetEventNotificationMode(JVMTI_ENABLE, JVMTI_EVENT_VM_INIT, NULL);
    jvmti->SetEventNotificationMode(JVMTI_ENABLE, JVMTI_EVENT_EXCEPTION, NULL);

    return 0;
}

将其编译为共享库并使用-agentpath选项运行java :

java -agentpath:/pato/to/libRichNPE.so Main