Android - JNI guidelines

问题

I want to incorporate small, lean and mean C-based parser into my Android project. I've done JNI programming in the past but not any type of native (C) development on Android. My plan is to compile C lib into SO and create JNI wrapper around it which I'm going to use in my app. Is this how it can/should be done? Second and most important question - how can I include .so into my APK? Where would it go?

回答1:

use Android NDK
Download n docs Android NDK 1.6

This will save you from writing JNI layer for lib and also will install the app in the lib folder of your app data folder.

回答2:

Depending on how much data you pass and how often I seriously doubt a Java/JNI/C would perform faster than a native java implementation.

Passing anything other than a "Java Int" to a "C long" invokes the JNI data conversion routines which are anything but lean and mean.

So unless your JNI routine fits the pattern:

Pass small amount of data.
Do lots and lots of work in C.
Pass small result set back.

You will be considerably slower than a native java implementation. If you stick with the basic "C" like java operations (+,-,*,==,>) using the "native" data types (int, char, String) and avoid the fancy libraries Java will perform nearly as fast as C.

The remaining bug-bear of java performance is the time taken to fire up the JVM and get everything going, but as you are starting off from a Java program this is a non issue.

The other reason for "slow" java performance is people insist on unnecessary factories, containers, xml beans etc. etc. where plain, simple methods would do the job better.

回答3:

Create a JNI folder in your Android root Application folder(where there is src, res) .Place the code (1) there name it as someshared-lib.c.

(1)

Java_YourPackageName_YourClassNameWhereYoudeclareYourNativeFunction_NativeFuntionName(JNIEnv* env,jobject thiz)
{


//your c code , the JNI will act as a wrapper for it

return (*env)->NewStringUTF(env, "<string to pass or you can mention jchar * type string >");

}

(2)IN java file

package YourPackageName;

public class YourClassNameWhereYoudeclareYourNativeFunction extends Activity
{

    public native String  NativeFuntionName();
    String returnValue = NativeFuntionName();


}

(3)IN Android.mk do this :

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := someshared-lib //note the libname same as c file name

LOCAL_SRC_FILES := someshared-lib.c //this is the file where you placed the code (1)

include $(BUILD_SHARED_LIBRARY)

export your ndk-build (do export PATH=:$PATH

go to the JNI folder as created above : execute ndk-build command

you will get a library formed someshared-lib in the lib folder formed in the Application root folder.While building and running the application this will get bundled up with the apk and will get installed in the device.To verify this you can go to the

/data/data/your_package_name/lib folder.

The app searched this lib in the /data/data/your_package_name/lib ( also /system/lib as well ) folder and use it for the dynamic calls(JNI) being made from the Android application.

Now if you want to return anything other than string you have to change the above method declration in c file as below :

Java_YourPackageName_YourClassNameWhereYoudeclareYourNativeFunction_NativeFuntionName(JNIEnv* env,jclass obj,jobject thiz)
{
jfieldID fid;
jboolean enable_flag;

//Pass the class object having all the variable from the android app to  the JNI in the jclass obj and access its members using the field ID and using Get and Set firld ID.

clazz = (*env)->GetObjectClass(env, info);
fid = (*env)->GetFieldID(env,clazz,"some_variable","X"); //"X" - type of variable for boolean it //is Z , for INtezer it is I, for double it is D,

refer this document for detailed explaination

//for getting teh value fomr the JNI
 enable_flag = (*env)->GetBooleanField(env, **thiz**, fid);


//for setting the value 

fid = (*env)->GetFieldID(env,clazz,"other_float_variable","D");
(*env)->SetDoubleField(env,**thiz**,fid,other_float_variable);



}

Also in the Android Application you have to pass the Class object of the structure. e.g

(2) will become now :

package YourPackageName;

public class YourClassNameWhereYoudeclareYourNativeFunction extends Activity
{

    public native String  NativeFuntionName();
    String returnValue = NativeFuntionName( exampleStruct exst);

where exampleStruct :

public class exampleStruct {

protected boolean  some_variable = 0;//no log saving by default 
protected float  other_float_variable   = 0;

}

Hope this helps.

回答4:

Someone said that JNI in android sucks : http://www.koushikdutta.com/2009/01/jni-in-android-and-foreword-of-why-jni.html

来源：https://stackoverflow.com/questions/1541884/android-jni-guidelines

标签

android

java-native-interface