Question

我正在使用JVMTI为Java应用程序编写一个检测工具。我已经看到JVMTI检测到抛出异常的时间以及根据http://docs.oracle.com/javase/7/docs/platform/jvmti/jvmti.html#Exception捕获的时间。本文档说明了Exception和ExceptionCatch事件

exception字段标识抛出的异常对象。

虽然它没有说明如何在运行期间比较它们（即比较Exception中提供的异常对应于ExceptionCatch中捕获的expcetion）。换句话说，对于

# java -version
java version "1.7.0_85"
OpenJDK Runtime Environment (IcedTea 2.6.1) (7u85-2.6.1-5ubuntu0.14.04.1)
OpenJDK 64-Bit Server VM (build 24.85-b03, mixed mode)

直接比较jexception / jobject似乎并不总是如此。考虑监视Exception和ExceptionCatch事件的JVMTI代理（下面的源代码），还考虑抛出异常的后续Java天真示例（也包括源代码），最后您可以通过“make run”运行代理示例“使用给定的Makefile（包含在最后）。

如果我使用OpenJDK 7运行该示例，它会给我以下结果：

# make run
...
cb_Exception (exception=0x2ae6b8087be8)
cb_ExceptionCatch (exception=0x2ae6b80859f8 vs last_exception=0x2ae6b8087be8)
AreSameObject? = 0
cb_Exception (exception=0x2ae6b80859f8)
cb_ExceptionCatch (exception=0x2ae6b807a388 vs last_exception=0x2ae6b80859f8)
AreSameObject? = 0
cb_Exception (exception=0x2ae6b807a388)
cb_ExceptionCatch (exception=0x2ae6b807a388 vs last_exception=0x2ae6b807a388)
AreSameObject? = 1
cb_Exception (exception=0x2ae6b807a388)
cb_ExceptionCatch (exception=0x2ae6b8078108 vs last_exception=0x2ae6b807a388)
AreSameObject? = 0
cb_Exception (exception=0x2ae6b8078108)
cb_ExceptionCatch (exception=0x2ae6b8078108 vs last_exception=0x2ae6b8078108)
AreSameObject? = 1
cb_Exception (exception=0x2ae6b8078108)
cb_ExceptionCatch (exception=0x2ae6b8078108 vs last_exception=0x2ae6b8078108)
AreSameObject? = 1
before doing work
cb_Exception (exception=0x2ae6b8078108)
cb_ExceptionCatch (exception=0x2ae6b8078108 vs last_exception=0x2ae6b8078108)
AreSameObject? = 1
after doing work

如果我使用IBM的Java 1.7.0运行该示例，情况有点类似。

# make run
...
cb_Exception (exception=0x7d78a0)
cb_ExceptionCatch (exception=0x7d7950 vs last_exception=0x7d78a0)
AreSameObject? = 1
cb_Exception (exception=0x7d7938)
cb_ExceptionCatch (exception=0x7d7950 vs last_exception=0x7d7938)
AreSameObject? = 0
cb_Exception (exception=0x7d7938)
cb_ExceptionCatch (exception=0x7d79a8 vs last_exception=0x7d7938)
AreSameObject? = 1
before doing work
cb_Exception (exception=0x7d7a60)
cb_ExceptionCatch (exception=0x7d7a98 vs last_exception=0x7d7a60)
AreSameObject? = 0
after doing work

在OpenJDK中，请注意每个Exception都有一个相应的ExceptionCatch，但主Java代码之外有六个异常，我怀疑它来自Java VM本身。请注意，异常的值在每对中都不相同。在第5次回调调用之后，异常的值似乎变得不变。这种情况在IBM的Java中有些类似，但引发的异常较少。作为来自评论的Chen Harel的建议，我已经存储了引发的异常，并通过IsSameObject JNI的方法与捕获的异常进行了比较，但JNI声称异常并不相同。

那么，Exception和ExceptionCatch可以在应用程序生命周期中识别异常吗？如果是，那么通过JVMTI或JNI比较异常的适当方法是什么？

agent.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <jni.h>
#include <jvmti.h>

#define CHECK_JVMTI_ERROR(x,call) \
    { if (x != JVMTI_ERROR_NONE) { fprintf (stderr, "Error during %s in %s:%d\n", #call, __FILE__, __LINE__); } }

/* Global static data */
static jvmtiEnv     *jvmti;
static jrawMonitorID ExtraeJ_AgentLock;

jobject last_exception;

static void JNICALL cb_Exception (jvmtiEnv *jvmti_env, JNIEnv* jni_env,
    jthread thread, jmethodID method, jlocation location, jobject exception,
    jmethodID catch_method, jlocation catch_location)
{
    printf ("cb_Exception (exception=%p)\n", exception);

    last_exception = exception;
}

static void JNICALL cb_ExceptionCatch (jvmtiEnv *jvmti_env,
    JNIEnv* jni_env, jthread thread, jmethodID method, jlocation location,
    jobject exception)
{
    printf ("cb_ExceptionCatch (exception=%p vs last_exception=%p)\n"
      "AreSameObject? = %d\n",
      exception,
      last_exception,
      (*jni_env)->IsSameObject(jni_env, exception, last_exception));
}

JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *vm, char *options, void *reserved)
{
    jint                rc;
    jvmtiError          r;
    jvmtiCapabilities   capabilities;
    jvmtiEventCallbacks callbacks;

    /* Get JVMTI environment */
    rc = (*vm)->GetEnv(vm, (void **)&jvmti, JVMTI_VERSION);
    if (rc != JNI_OK)
    {
        fprintf (stderr, "Error!: Unable to create jvmtiEnv, rc=%d\n", rc);
        return -1;
    }

    /* Get/Add JVMTI capabilities */
    memset(&capabilities, 0, sizeof(capabilities));
    capabilities.can_generate_exception_events = 1;
    r = (*jvmti)->AddCapabilities(jvmti, &capabilities);
    CHECK_JVMTI_ERROR(r, AddCapabilities);

    /* Set callbacks and enable event notifications */
    memset(&callbacks, 0, sizeof(callbacks));
    callbacks.Exception               = &cb_Exception;
    callbacks.ExceptionCatch          = &cb_ExceptionCatch;
    r = (*jvmti)->SetEventCallbacks(jvmti, &callbacks, sizeof(callbacks));
    CHECK_JVMTI_ERROR(r, SetEventCallbacks);

    /* Exception events */
    r = (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE, 
      JVMTI_EVENT_EXCEPTION, NULL);
    CHECK_JVMTI_ERROR(r, SetEventNotificationMode);
    r = (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE, 
      JVMTI_EVENT_EXCEPTION_CATCH, NULL);
    CHECK_JVMTI_ERROR(r, SetEventNotificationMode);

    return 0;
}

JNIEXPORT void JNICALL Agent_OnUnload(JavaVM *vm)
{
}

example.java

public class example
{
    void except () throws Exception
    { throw new Exception ("new-exception"); }

    void do_work()
    {
        System.out.println ("before doing work");

        try
        { except(); }
        catch (Exception e)
        {}

        System.out.println ("after doing work");
    }

    public static void main (String [] args)
    {
        example e = new example();
        e.do_work ();
    }
}

生成文件

JAVA_JDK=/usr/lib/jvm/java-7-openjdk-amd64

all: libagent.so example.class

libagent.so: agent.c
    gcc -shared -fPIC -DPIC agent.c -o libagent.so -I$(JAVA_JDK)/include

example.class: example.java
    javac example.java

run: libagent.so example.class
    java -agentpath:$(PWD)/libagent.so example

11月9日更新

我已经像Chen Harel所建议的那样创建了对异常的全局引用。 agent.c的修改版本如下所示，执行的输出如下所示。

agent.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <jni.h>
#include <jvmti.h>

#define CHECK_JVMTI_ERROR(x,call) \
    { if (x != JVMTI_ERROR_NONE) { fprintf (stderr, "Error during %s in %s:%d\n", #call, __FILE__, __LINE__); } }

/* Global static data */
static jvmtiEnv     *jvmti;
static jrawMonitorID ExtraeJ_AgentLock;

jobject last_exception;

static void JNICALL cb_Exception (jvmtiEnv *jvmti_env, JNIEnv* jni_env,
    jthread thread, jmethodID method, jlocation location, jobject exception,
    jmethodID catch_method, jlocation catch_location)
{
    printf ("cb_Exception (exception=%p)\n", exception);

    last_exception = (*jni_env)->NewGlobalRef (jni_env, exception);
}

static void JNICALL cb_ExceptionCatch (jvmtiEnv *jvmti_env,
    JNIEnv* jni_env, jthread thread, jmethodID method, jlocation location,
    jobject exception)
{
    printf ("cb_ExceptionCatch (exception=%p vs last_exception=%p)\n"
      "AreSameObject? = %d\n",
      exception,
      last_exception,
      (*jni_env)->IsSameObject(jni_env, exception, last_exception));

    (*jni_env)->DeleteGlobalRef(jni_env, last_exception);
}

JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *vm, char *options, void *reserved)
{
    jint                rc;
    jvmtiError          r;
    jvmtiCapabilities   capabilities;
    jvmtiEventCallbacks callbacks;

    /* Get JVMTI environment */
    rc = (*vm)->GetEnv(vm, (void **)&jvmti, JVMTI_VERSION);
    if (rc != JNI_OK)
    {
        fprintf (stderr, "Error!: Unable to create jvmtiEnv, rc=%d\n", rc);
        return -1;
    }

    /* Get/Add JVMTI capabilities */
    memset(&capabilities, 0, sizeof(capabilities));
    capabilities.can_generate_exception_events = 1;
    r = (*jvmti)->AddCapabilities(jvmti, &capabilities);
    CHECK_JVMTI_ERROR(r, AddCapabilities);

    /* Set callbacks and enable event notifications */
    memset(&callbacks, 0, sizeof(callbacks));
    callbacks.Exception               = &cb_Exception;
    callbacks.ExceptionCatch          = &cb_ExceptionCatch;
    r = (*jvmti)->SetEventCallbacks(jvmti, &callbacks, sizeof(callbacks));
    CHECK_JVMTI_ERROR(r, SetEventCallbacks);

    /* Exception events */
    r = (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE, 
      JVMTI_EVENT_EXCEPTION, NULL);
    CHECK_JVMTI_ERROR(r, SetEventNotificationMode);
    r = (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE, 
      JVMTI_EVENT_EXCEPTION_CATCH, NULL);
    CHECK_JVMTI_ERROR(r, SetEventNotificationMode);

    return 0;
}

JNIEXPORT void JNICALL Agent_OnUnload(JavaVM *vm)
{
}

** make run **

的输出

# make run
...
cb_Exception (exception=0x2b13b0087c18)
cb_ExceptionCatch (exception=0x2b13b0085a08 vs last_exception=0x2b13e4001608)
AreSameObject? = 0
cb_Exception (exception=0x2b13b0085a08)
cb_ExceptionCatch (exception=0x2b13b007a388 vs last_exception=0x2b13e4001610)
AreSameObject? = 0
cb_Exception (exception=0x2b13b007a388)
cb_ExceptionCatch (exception=0x2b13b007a388 vs last_exception=0x2b13e4001618)
AreSameObject? = 1
cb_Exception (exception=0x2b13b007a388)
cb_ExceptionCatch (exception=0x2b13b0078108 vs last_exception=0x2b13b0085a00)
AreSameObject? = 0
cb_Exception (exception=0x2b13b0078108)
cb_ExceptionCatch (exception=0x2b13b0078108 vs last_exception=0x2b13b0085a08)
AreSameObject? = 1
cb_Exception (exception=0x2b13b0078108)
cb_ExceptionCatch (exception=0x2b13b0078108 vs last_exception=0x2b13b0085a10)
AreSameObject? = 1
before doing work
cb_Exception (exception=0x2b13b0078108)
cb_ExceptionCatch (exception=0x2b13b0078108 vs last_exception=0x2b13b0085a18)
AreSameObject? = 1
after doing work

Answer 1

jobject是一个C ++指针，在它下面处理对堆的引用。因此，如果您考虑它，这更像是指针的指针。

测试两个jobject是否相同的方法是使用jni方法IsSameObject

如果要测试异常的类型，请使用GetObjectClass + IsInstanceOf

修改

请注意，为了在方法之间保持jobject（s）有效，您必须使用jni NewGlobalRef方法为它们创建引用。

通过JVMTI

1 个答案: