通常使用java JNI、GetFieldID和JNI。

Question

我在JNI上的运气似乎一点也不好。我在等我买的一本书送到，但现在还在反复试验。

我正在使用JNI实现一个Lua赋值器。evaluatorNew()只是创建一个新的Evaluator()对象，按照Java端的描述创建一个新的CPtr()类对象，将我们的Evaluator()对象的指针分配给CPtr的'peer‘成员，然后返回新的CPtr jobject。

JNIEXPORT jobject JNICALL Java_com_starlon_libscriptable_UtilsEvaluator_evaluatorNew(
    JNIEnv *env, jclass clazz)
{
    int foo = 1;
    while(foo);

    Evaluator *eval = new Evaluator();

    jobject obj;
    jclass tempClass;

    tempClass = env->FindClass("com/starlon/libscriptable/CPtr");

    obj = env->AllocObject( tempClass );
    if (obj)
    {
        env->SetLongField( obj, env->GetFieldID( tempClass, "peer", "J" ), (jlong)eval);
    }
    return obj;
}

下面是用来保存指针的Java类。这就是'env->FindClass()‘调用的内容。

public class CPtr
{
    /* Pointer value of the real C pointer. Use long to be 64-bit safe. */
    private long peer;

    protected long getPeer()
    {
        return peer;
    }

    /* No-args constructor. */
    CPtr() {}

}

正如您在下面看到的，没有堆栈跟踪可言。

(gdb) bt
#0  0xafd15ca8 in __libc_android_abort ()
   from /home/scott/workspace/StarVisuals/StarVisuals/obj/local/armeabi-v7a/libc.so
#1  0x00000008 in ?? ()
Backtrace stopped: frame did not save the PC

下面是gdb单步执行上面的JNI函数的转储：

0x80805258 in Java_com_starlon_libscriptable_UtilsEvaluator_evaluatorNew (env=0xabd8, 
    clazz=0x40715698) at jni/libscriptable//evaluator.cpp:85
85      while(foo);
Current language:  auto; currently c++
(gdb) set foo = 0
(gdb) s
[New Thread 11226]
[Switching to Thread 11226]
87      Evaluator *eval = new Evaluator();
(gdb) u
92      tempClass = env->FindClass("com/starlon/libscriptable/CPtr");
(gdb) u
94      obj = env->AllocObject( tempClass );
(gdb) u
95      if (obj)
(gdb) s
97          env->SetLongField( obj, env->GetFieldID( tempClass, "peer", "J" ), (jlong)eval);
(gdb) s
_JNIEnv::GetFieldID (this=0xabd8, clazz=0x40715910, name=0x8080a134 "peer", sig=0x8080a13c "J")
    at /opt/android-ndk-r7b/platforms/android-9/arch-arm/usr/include/jni.h:708
708     { return functions->GetFieldID(this, clazz, name, sig); }
(gdb) print functions
$1 = (const JNINativeInterface *) 0xaca9dd68
(gdb) print this
$2 = (_JNIEnv * const) 0xabd8
(gdb) print clazz
$3 = (jclass) 0x40715910
(gdb) print name
$4 = 0x8080a134 "peer"
(gdb) print sig
$5 = 0x8080a13c "J"
(gdb) print *functions
$6 = {reserved0 = 0x0, reserved1 = 0x0, reserved2 = 0x0, reserved3 = 0x0, GetVersion = 0xaca43385, 
  DefineClass = 0xaca43355, FindClass = 0xaca4773d, FromReflectedMethod = 0xaca47719, 
  FromReflectedField = 0xaca476f5, ToReflectedMethod = 0xaca476b9, GetSuperclass = 0xaca44801, 
  IsAssignableFrom = 0xaca4450d, ToReflectedField = 0xaca4767d, Throw = 0xaca43331, 
  ThrowNew = 0xaca47655, ExceptionOccurred = 0xaca447b1, ExceptionDescribe = 0xaca4761d, 
  ExceptionClear = 0xaca43315, FatalError = 0xaca44d99, PushLocalFrame = 0xaca475c1, 
  PopLocalFrame = 0xaca47561, NewGlobalRef = 0xaca44ac1, DeleteGlobalRef = 0xaca4502d, 
---Type <return> to continue, or q <return> to quit---q
SetByteFieldQuit
(gdb) print functions->GetFieldID
$7 = (jfieldID (*)(JNIEnv *, jclass, const char *, const char *)) 0xaca47435
(gdb) s

Program received signal SIGSEGV, Segmentation fault.
0xaca4cd0c in ?? ()
(gdb) bt
#0  0xafd15ca8 in __libc_android_abort ()
   from /home/scott/workspace/StarVisuals/StarVisuals/obj/local/armeabi-v7a/libc.so
#1  0x00000008 in ?? ()
Backtrace stopped: frame did not save the PC
(gdb) quit

下面是jni.h的第708行：

jfieldID GetFieldID(jclass clazz, const char* name, const char* sig)
{ return functions->GetFieldID(this, clazz, name, sig); } // 708

有什么帮助吗？如果我需要澄清什么，请告诉我。

Answer 1

AllocObject分配对象，但不构造对象。我怀疑如果你在一个未构造的对象上调用SetLongField，它可能会吐得很厉害，就像你在这里所做的那样。请改用NewObject；首先必须查询无参数构造函数的方法。

Answer 2

你的代码中有很多是理所当然的。你需要检查每一个JNI调用的结果，寻找一个异常，如果你得到一个异常，就中止。你不应该像那样堆叠Lisp风格的函数调用，并希望它们都返回可用的值。

Answer 3

您应该始终使用CheckJNI运行：http://android-developers.blogspot.com/2011/07/debugging-android-jni-with-checkjni.html

它会告诉您正在调用JNI函数，但异常挂起，并干净利落地中止了。