Implementation of JNI in Android system

Implementation of JNI in Android system
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Implementation of JNI in Android system

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JNI (Java Native Interface) defines a method for Java code to call other Code such as C or C ++ code.

In Android, JNI is described through the JNINativeMethod struct, which is defined in jni. h as follows:

typedef struct {    const char* name;    const char* signature;    void*       fnPtr;} JNINativeMethod;

The first parameter name is the function name in Java code.

The second parameter signature is used to describe the parameters and return values of a function.

The third parameter fnPtr is the pointer of the function in C code.

The second parameter is a string that describes the function parameters and return values. The format of the string is as follows:

(XX...) X

The value and definition of X are as follows:

Character	Java type	C type
V	Void	Void
Z	Jboolean	Unsigned char
B	Jbyte	Signed char
C	Jchar	Unsigned short
S	Jshort	Short
I	Jint	Int
J	Jlong	Long
F	Jfloat	Float
D	Jdouble	Double

In addition, the meaning of these characters can also be seen from the definition of the variable type in jni. h, as shown below:

typedef unsigned char   jboolean;       /* unsigned 8 bits */typedef signed char     jbyte;          /* signed 8 bits */typedef unsigned short  jchar;          /* unsigned 16 bits */typedef short           jshort;         /* signed 16 bits */typedef int             jint;           /* signed 32 bits */typedef long long       jlong;          /* signed 64 bits */typedef float           jfloat;         /* 32-bit IEEE 754 */typedef double          jdouble;        /* 64-bit IEEE 754 */typedef union jvalue {    jboolean    z;    jbyte       b;    jchar       c;    jshort      s;    jint        i;    jlong       j;    jfloat      f;    jdouble     d;    jobject     l;} jvalue;

For example, add a HAL layer to a driver and create a JNI layer. For the JNI layer only, create the frameworks/base/services/jni/com_android_server_DemoService.cpp file. The code used to describe the JNI interface is as follows:

static const JNINativeMethod method_table[] = {                {"init_native", "()I", (void*)demo_init},                {"setVal_native", "(II)V", (void*)demo_setVal},                {"getVal_native", "(I)I", (void*)demo_getVal},        };

"(II) V" indicates that the function has two integer parameters, and the return value is void.

Note: The function parameters and return value types specified by parameter 2 must be consistent with the C function parameters and return values. Otherwise, although compilation can pass, during the Android system loading process, the following error is reported, causing the Android system to fail.

E/dalvikvm( 1737): ERROR: couldn't find native methodE/dalvikvm( 1737): Requested: Lcom/android/server/DemoService;.init_native:()ZE/dalvikvm( 1737): Candidate: Lcom/android/server/DemoService;.init_native:()IE/JNIHelp ( 1737): RegisterNatives failed for 'com/android/server/DemoService', abortingF/libc    ( 1737): Fatal signal 11 (SIGSEGV) at 0xdeadbaad (code=1), thread 1737 (system_server)

The above error occurs in the JNI_OnLoad () function in the frameworks/base/services/jni/onload. cpp file, as shown below:

extern "C" jint JNI_OnLoad(JavaVM* vm, void* reserved){    JNIEnv* env = NULL;    jint result = -1;    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {        ALOGE("GetEnv failed!");        return result;    }    ALOG_ASSERT(env, "Could not retrieve the env!");    ……       register_android_server_DemoService(env);    ……    return JNI_VERSION_1_4;}

In addition, ProGuard program optimization may also lead to the above running errors. In this case, you can add the "LOCAL_PROGUARD_ENABLED: = disabled" macro to the makefile file to disable ProGuard optimization.

For ProGuard, refer to its official website: http://proguard.sourceforge.net/

ProGuard is a free Java class file compressors, optimizer, obfuscators, and pre-validators. It detects and deletes unused classes, fields, methods, and attributes. It optimizes bytecode to the maximum extent and deletes useless commands. It renames the remaining classes, fields, and methods with a very short meaningless name. Finally, it performs pre-verification on the processed code.

ProGuard has the following functions:

A. Create more compact code for smaller code files, Faster network transmission, faster loading speed, and smaller memory usage;

B. Make reverse engineering difficult for programs and libraries;

C. List dead code so that it can be deleted;

D. relocate and pre-validate existing class files for Java 6 or later versions to make full use of the class loading performance.