android with jni android studio version: 2.1.1 gradle version: 2.1.0

1. JNI 使用淺談
阿胖 2016.09.29
Android Taipei

2. Outline
1. Java call c
2. c call java
3. c vs cpp
4. Data Type Mapping
5. Data Type Convert
6. JNIEnv

3. Java call c
HelloJni.java
hello-jni.c

4. HelloJni.java
public native String stringFromJNI();
static {
System.loadLibrary("hello-jni");
}

5. hello-jni.c
jtype Java_PATH_NAME_CLASSNAME_FUNCTIONNAME(
JNIEnv* env, jobject thiz [,… jparams] )

6. hello-jni.c
jstring
Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env, jobject thiz )
{
char msg[] = "Hello World from JNI !";
jstring jMsg = (*env)->NewStringUTF(env, msg);
return jMsg;
}

7. c call java
HelloJni.java
hello-jni.c

8. HelloJni.java
private void callFromJni(String msg) {
Log.i(TAG,"123 "+msg);
}

9. hello-jni.c
NativeType Call<type>Method(JNIEnv *env, jobject obj, jmethodID methodID, ...);
NativeType Call<type>MethodA(JNIEnv *env, jobject obj, jmethodID methodID, const jvalue *args);
NativeType Call<type>MethodV(JNIEnv *env, jobject obj, jmethodID methodID, va_list args);
jmethodID GetMethodID(JNIEnv *env, jclass clazz, const char *name, const char *sig);
jclass GetObjectClass(JNIEnv *env, jobject obj);

10. hello-jni.cpp
jstring Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env, jobject thiz ) {
char msg[] = "Hello World from JNI !";
jstring jMsg = env->NewStringUTF(msg);
return jMsg;
}
jclass jClass = env->GetObjectClass(thiz);
jmethodID mid = env->GetMethodID(jClass, "callFromJni", "(Ljava/lang/String;)V" );
env->CallVoidMethod(thiz, mid, jMsg);

11. c vs cpp
jstring jMsg = (*env)->NewStringUTF(env, msg);
jstring jMsg = env->NewStringUTF(msg);
#ifdef __cplusplus
extern "C" {
#endif
#ifdef __cplusplus
}
#endif

12. Data Type Mapping
Java primitive data type Native primitive data type Description
void void None
byte jbyte 8-bit signed. Range is -2^7 to 2^7 - 1
int jint 32-bit signed. Range is -2^31 to 2^31 - 1
float jfloat 32 bits. Represent a real number as small as 1.4 x 10-45
and as big as 3.4 x 1038 (approx.), positive or negative
double jdouble 64 bits. Represent a real number as small as 4.9 x 10-324
and as big as 1.7 x 10308 (approx.), positive or negative
char jchar 16-bit unsigned. Range is 0 to 65535
long jlong 64-bit signed. Range -2^63 to 2^63 - 1
short jshort 16-bit signed. Range is -2^15 to 2^15 - 1
boolean jboolean Unsigned 8 bits. true and false

13. Data Type Mapping
Java Reference type JNI type Description
java.lang.object jobject Any Java object
java.lang.String jstring String representation
java.lang.Class jclass Java class object
java.lang.Throwable jthrowable Java throwable object

14. Data Type Mapping
Type Signature Java Type
Z boolean
B byte
C char
S short
I int
J long
F float
D double
V void
[ []
L<packet>/object; object

15. Data Type Mapping
Function in java signature
int foo() “()I”
void foo(byte[] arg) “([B)V”
boolean foo(String arg) “(Ljava/lang/String;)Z”
"(parameterFieldDescriptor)returnFieldDescriptor"

16. Data Type Convert
NewStringUTF
jstring NewStringUTF(JNIEnv *env, const char *bytes);
jstring jstr = (*env)->NewStringUTF(env, "This comes from jni.");
GetStringUTFChars
const char * GetStringUTFChars(JNIEnv *env, jstring string, jboolean *isCopy);
const char* str = (*env)->GetStringUTFChars(env, jstr , NULL);
ReleaseStringUTFChars
void ReleaseStringUTFChars(JNIEnv *env, jstring string, const char *utf);
String

17. Data Type Convert
New<PrimitiveType>Array Routines
ArrayType New<PrimitiveType>Array(JNIEnv *env, jsize length);
Set<PrimitiveType>ArrayRegion Routines
void Set<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType array,
jsize start, jsize len, const NativeType *buf);
Array

18. Data Type Convert
Get<PrimitiveType>ArrayElements Routines
NativeType *Get<PrimitiveType>ArrayElements(JNIEnv *env,
ArrayType array, jboolean *isCopy);
Release<PrimitiveType>ArrayElements Routines
void Release<PrimitiveType>ArrayElements(JNIEnv *env,
ArrayType array, NativeType *elems, jint mode);
Get<PrimitiveType>ArrayRegion Routines
void Get<PrimitiveType>ArrayRegion(JNIEnv *env, ArrayType array,
jsize start, jsize len, NativeType *buf);
Array

19. Data Type Convert
struct{
char name[256];
int num;
} Foo;
Object
class Foo {
public String name;
public int num;
}

20. Data Type Convert
FindClass
jclass FindClass(JNIEnv *env, const char *name);
“your/java/path/name/class_name$inner_class_name”
jmethodID mid = env->GetMethodID(msgClass, "<init>", "()V" );
NewObject
jobject NewObject(JNIEnv *env, jclass clazz, jmethodID methodID, ...);
GetFieldID
jfieldID GetFieldID(JNIEnv *env, jclass clazz, const char *name, const char *sig);
Set<type>Field Routines
void Set<type>Field(JNIEnv *env, jobject obj, jfieldID fieldID, NativeType value);
Get<type>Field Routines
Object

21. Data Type Convert
NewGlobalRef
jobject NewGlobalRef(JNIEnv *env, jobject obj);
DeleteGlobalRef
void DeleteGlobalRef(JNIEnv *env, jobject globalRef);
DeleteLocalRef
void DeleteLocalRef(JNIEnv *env, jobject localRef);
static_cast<type>

22. JNIEnv
jint JNI_OnLoad(JavaVM* vm, void* reserved)
{
LOGI("JNI_OnLoad");
JNIEnv* env = NULL;
jint result = -1;
if(vm->GetEnv((void**)&env, JNI_VERSION_1_4) != JNI_OK) {
return result;
}
jvm = vm;
return JNI_VERSION_1_4;
}

23. JNIEnv* getJNIEnv() {
JNIEnv* env = NULL;
int res = jvm->GetEnv((void**)&env, JNI_VERSION_1_4);
if (res == JNI_EDETACHED) {
//LOGI("GetEnv: not attached");
if (jvm->AttachCurrentThread(&env, NULL) != 0) {
LOGI("Failed to attach");
return NULL;
}
} else if (res == JNI_OK) {
//
} else if (res == JNI_EVERSION) {
LOGI("GetEnv: version not supported");
return NULL;
}
return env;
}

24. JNIEnv
JNIEnv* env = getJNIEnv();
/* do something */
jvm->DetachCurrentThread();

25. Reference
http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/functions.html
http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/types.html
http://www.developer.com/java/data/jni-data-type-mapping-to-cc.html
source code : https://github.com/kentpon/hello-jni

26. QA