进行 JNI 开发的时候，JNI 的 native 方法第一个入参为 JNIEnv,这个 JNIEnv 实质上的JNINativeInterface*
//@jni.h
struct _JNIEnv;
struct _JavaVM;
typedef const struct JNINativeInterface* C_JNIEnv;
#if defined(__cplusplus)
typedef _JNIEnv JNIEnv;
typedef _JavaVM JavaVM;
#else
typedef const struct JNINativeInterface* JNIEnv;
typedef const struct JNIInvokeInterface* JavaVM;
#endif
...
jclass FindClass(const char* name){
return functions->FindClass(this, name);
}
...
形如CallStaticVoidMethod 的有返回值的方法，是宏定义的方法。而对于 CallStaticVoidMethod则是直接定义的。同时像GetStaticFieldID也都是再次定义。可以说这里申明了 JNI 的方法表。
#define CALL_STATIC_TYPE_METHOD(_jtype, _jname) \
_jtype CallStatic##_jname##Method(jclass clazz, jmethodID methodID, \
...) \
{ \
_jtype result; \
va_list args; \
va_start(args, methodID); \
result = functions->CallStatic##_jname##MethodV(this, clazz, \
methodID, args); \
va_end(args); \
return result; \
}
#define CALL_STATIC_TYPE_METHODV(_jtype, _jname) \
_jtype CallStatic##_jname##MethodV(jclass clazz, jmethodID methodID, \
va_list args) \
{ return functions->CallStatic##_jname##MethodV(this, clazz, methodID, \
args); }
#define CALL_STATIC_TYPE_METHODA(_jtype, _jname) \
_jtype CallStatic##_jname##MethodA(jclass clazz, jmethodID methodID, \
jvalue* args) \
{ return functions->CallStatic##_jname##MethodA(this, clazz, methodID, \
args); }
#define CALL_STATIC_TYPE(_jtype, _jname) \
CALL_STATIC_TYPE_METHOD(_jtype, _jname) \
CALL_STATIC_TYPE_METHODV(_jtype, _jname) \
CALL_STATIC_TYPE_METHODA(_jtype, _jname)
CALL_STATIC_TYPE(jobject, Object)
CALL_STATIC_TYPE(jboolean, Boolean)
CALL_STATIC_TYPE(jbyte, Byte)
CALL_STATIC_TYPE(jchar, Char)
CALL_STATIC_TYPE(jshort, Short)
CALL_STATIC_TYPE(jint, Int)
CALL_STATIC_TYPE(jlong, Long)
CALL_STATIC_TYPE(jfloat, Float)
CALL_STATIC_TYPE(jdouble, Double)
void CallStaticVoidMethod(jclass clazz, jmethodID methodID, ...)
{
va_list args;
va_start(args, methodID);
functions->CallStaticVoidMethodV(this, clazz, methodID, args);
va_end(args);
}
void CallStaticVoidMethodV(jclass clazz, jmethodID methodID, va_list args)
{ functions->CallStaticVoidMethodV(this, clazz, methodID, args); }
void CallStaticVoidMethodA(jclass clazz, jmethodID methodID, jvalue* args)
{ functions->CallStaticVoidMethodA(this, clazz, methodID, args); }
从上面也能看到， CallStatic##_jname##Method会调用CallStatic##_jname##MethodV。
同时这些方法都会转调到 functions 的同名方法。各同名方法指针在
@Jni.cpp
static const struct JNINativeInterface gNativeInterface
其中的 CallStaticVoidMethodV也都是宏定义函数
@Jni.cpp
/*
* Call a static method.
*/
#define CALL_STATIC(_ctype, _jname, _retfail, _retok, _isref) \
static _ctype CallStatic##_jname##Method(JNIEnv* env, jclass jclazz, \
jmethodID methodID, ...) \
{ \
UNUSED_PARAMETER(jclazz); \
ScopedJniThreadState ts(env); \
JValue result; \
va_list args; \
va_start(args, methodID); \
dvmCallMethodV(ts.self(), (Method*)methodID, NULL, true, &result, args);\
va_end(args); \
if (_isref && !dvmCheckException(ts.self())) \
result.l = (Object*)addLocalReference(ts.self(), result.l); \
return _retok; \
} \
static _ctype CallStatic##_jname##MethodV(JNIEnv* env, jclass jclazz, \
jmethodID methodID, va_list args) \
{ \
UNUSED_PARAMETER(jclazz); \
ScopedJniThreadState ts(env); \
JValue result; \
dvmCallMethodV(ts.self(), (Method*)methodID, NULL, true, &result, args);\
if (_isref && !dvmCheckException(ts.self())) \
result.l = (Object*)addLocalReference(ts.self(), result.l); \
return _retok; \
} \
static _ctype CallStatic##_jname##MethodA(JNIEnv* env, jclass jclazz, \
jmethodID methodID, jvalue* args) \
{ \
UNUSED_PARAMETER(jclazz); \
ScopedJniThreadState ts(env); \
JValue result; \
dvmCallMethodA(ts.self(), (Method*)methodID, NULL, true, &result, args);\
if (_isref && !dvmCheckException(ts.self())) \
result.l = (Object*)addLocalReference(ts.self(), result.l); \
return _retok; \
}
CALL_STATIC(jobject, Object, NULL, (jobject) result.l, true);
CALL_STATIC(jboolean, Boolean, 0, result.z, false);
CALL_STATIC(jbyte, Byte, 0, result.b, false);
CALL_STATIC(jchar, Char, 0, result.c, false);
CALL_STATIC(jshort, Short, 0, result.s, false);
CALL_STATIC(jint, Int, 0, result.i, false);
CALL_STATIC(jlong, Long, 0, result.j, false);
CALL_STATIC(jfloat, Float, 0.0f, result.f, false);
CALL_STATIC(jdouble, Double, 0.0, result.d, false);
CALL_STATIC(void, Void, , , false);
宏替换
static void CallStaticVoidMethod(JNIEnv* env, jclass jclazz, \
jmethodID methodID, ...) \
{ \
UNUSED_PARAMETER(jclazz); \
ScopedJniThreadState ts(env); \
JValue result; \
va_list args; \
va_start(args, methodID); \
dvmCallMethodV(ts.self(), (Method*)methodID, NULL, true, &result, args);\
va_end(args); \
if (false && !dvmCheckException(ts.self())) \
result.l = (Object*)addLocalReference(ts.self(), result.l); \
return _retok; \
} \
static void CallStaticVoidMethodV(JNIEnv* env, jclass jclazz, \
jmethodID methodID, va_list args) \
{ \
UNUSED_PARAMETER(jclazz); \
ScopedJniThreadState ts(env); \
JValue result; \
dvmCallMethodV(ts.self(), (Method*)methodID, NULL, true, &result, args);\
if (false && !dvmCheckException(ts.self())) \
result.l = (Object*)addLocalReference(ts.self(), result.l); \
return _retok; \
} \
static void CallStaticVoidMethodA(JNIEnv* env, jclass jclazz, \
jmethodID methodID, jvalue* args) \
{ \
UNUSED_PARAMETER(jclazz); \
ScopedJniThreadState ts(env); \
JValue result; \
dvmCallMethodA(ts.self(), (Method*)methodID, NULL, true, &result, args);\
if (false && !dvmCheckException(ts.self())) \
result.l = (Object*)addLocalReference(ts.self(), result.l); \
return _retok; \
}
于是找到了实际调用点 dvmCallMethodV( );该方法定义在 Stack.h，实现于 Stack.cpp
/*
* Issue a method call with a variable number of arguments. We process
* the contents of "args" by scanning the method signature.
*
* Pass in NULL for "obj" on calls to static methods.
*
* We don't need to take the class as an argument because, in Dalvik,
* we don't need to worry about static synchronized methods.
*/
void dvmCallMethodV(Thread* self, const Method* method, Object* obj,
bool fromJni, JValue* pResult, va_list args)
{
const char* desc = &(method->shorty[1]); // [0] is the return type.
int verifyCount = 0;
ClassObject* clazz;
u4* ins;
clazz = callPrep(self, method, obj, false);
if (clazz == NULL)
return;
/* "ins" for new frame start at frame pointer plus locals */
ins = ((u4*)self->interpSave.curFrame) +
(method->registersSize - method->insSize);
//LOGD(" FP is %p, INs live at >= %p", self->interpSave.curFrame, ins);
/* put "this" pointer into in0 if appropriate */
if (!dvmIsStaticMethod(method)) {
#ifdef WITH_EXTRA_OBJECT_VALIDATION
assert(obj != NULL && dvmIsHeapAddress(obj));
#endif
*ins++ = (u4) obj;
verifyCount++;
}
while (*desc != '\0') {
switch (*(desc++)) {
case 'D': case 'J': {
u8 val = va_arg(args, u8);
memcpy(ins, &val, 8); // EABI prevents direct store
ins += 2;
verifyCount += 2;
break;
}
case 'F': {
/* floats were normalized to doubles; convert back */
float f = (float) va_arg(args, double);
*ins++ = dvmFloatToU4(f);
verifyCount++;
break;
}
case 'L': { /* 'shorty' descr uses L for all refs, incl array */
void* arg = va_arg(args, void*);
assert(obj == NULL || dvmIsHeapAddress(obj));
jobject argObj = reinterpret_cast(arg);
if (fromJni)
*ins++ = (u4) dvmDecodeIndirectRef(self, argObj);
else
*ins++ = (u4) argObj;
verifyCount++;
break;
}
default: {
/* Z B C S I -- all passed as 32-bit integers */
*ins++ = va_arg(args, u4);
verifyCount++;
break;
}
}
}
#ifndef NDEBUG
if (verifyCount != method->insSize) {
LOGE("Got vfycount=%d insSize=%d for %s.%s", verifyCount,
method->insSize, clazz->descriptor, method->name);
assert(false);
goto bail;
}
#endif
//dvmDumpThreadStack(dvmThreadSelf());
if (dvmIsNativeMethod(method)) {
TRACE_METHOD_ENTER(self, method);
/*
* Because we leave no space for local variables, "curFrame" points
* directly at the method arguments.
*/
(*method->nativeFunc)((u4*)self->interpSave.curFrame, pResult,
method, self);
TRACE_METHOD_EXIT(self, method);
} else {
dvmInterpret(self, method, pResult);
}
#ifndef NDEBUG
bail:
#endif
dvmPopFrame(self);
}