Android : Need to create Shared Preferences object in c++ NDK and Store some Boolean value

Question

I am new to this don't know how to start this,

I have created a project that is linked to C++ using Android.mk

So when I call a method from java it should do the storing boolean value to my shared preference object.

This is my JNI method

extern "C"
JNIEXPORT void JNICALL
Java_com_example_sample_storeBoolean(JNIEnv *env,jobject instance){
//TODO 
const char *name ="hello";
__android_log_print(ANDROID_LOG_ERROR, "TRACKERS", "***** %s *****", name);
}

normal log I have printed it is working now only just need create sharepreference object and set boolean value

SharedPreferences prefs = context.getSharedPreferences("myprefdata", Context.MODE_PRIVATE);

prefs.edit().putBoolean("settingnootification", true).commit();

Please guide me how to do. Thanks

public abstract SharedPreferences getSharedPreferences(String name, int mode);

Need to use this method in c++

Answer 1

I just called saveBoolean(boolean bool) in MainActivity from JNI and it saved the value. Here is code: MainActivity

public class MainActivity extends AppCompatActivity {

// Used to load the 'native-lib' library on application startup.
static {
    System.loadLibrary("native-lib");
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    stringFromJNI(this);
}

/**
 * A native method that is implemented by the 'native-lib' native library,
 * which is packaged with this application.
 */
public native void stringFromJNI(MainActivity mainActivity);

public void saveBoolean(boolean bool){
    SharedPreferences sharedPreferences = this.getSharedPreferences("Test", Context.MODE_PRIVATE);
    sharedPreferences.edit().putBoolean("testBool",bool).commit();
    Log.d("MainActivity","saveBoolean Called "+bool);
}

}

#include <jni.h>
#include <string>

extern "C"
JNIEXPORT void JNICALL
Java_com_android_techgig_sharedpref_MainActivity_stringFromJNI(JNIEnv *env,jobject  obj /* this */) {

    jclass cls = (env)->GetObjectClass(obj); //for getting class
    jmethodID mid = (env)->GetMethodID(cls, "saveBoolean", "(Z)V"); //for getting method signature, Z for boolean
    if (mid == 0)
        return;
    //will return 0 in case of class not found
    (env)->CallVoidMethod(obj, mid, true); //now calling actual method
    printf("native called");
}

Here is method signatures types

Signature   Java Type
Z   boolean
B   byte
C   char
S   short
I   int
J   long
F   float
D   double

Here is link to explore more ..

Happy coding!!!

Answer 2

I guess you want to call Java class from your code. I suggest to call Java back from your C++ code.

Take a look here:

https://github.com/mkowsiak/jnicookbook/tree/master/recipes/recipeNo032

In this sample, what you do is:

• Java calls C++
• Based on info passed from Java, C++ attaches to JVM
• C++ code calls Java code from another class (in your case, it will be SharedPreferences object)

It might be you will need some helper class to make things simpler. Eg PreferencesStorer - where you will get proper preferences class and pass values you want to store.

Take a look here:

public static int fun() {
  System.out.println("From JVM");
  return 0;
}

This is the method you want to call but you want it to be: store(String value, int mode).

In your code (in Java) you need to create this object and pass it as argument to your C++ code. Inside C++ you want to call method of this object. Inside JVM it will be already there - ready to be called.

If this is not what you are looking for, I think you need to provide some more info to make the whole use-case little bit more clear.

Have fun with JNI

Answer 3

I need to call a getSharedPreferences() method, So how can I call that and store boolean.

So, I have created two simple methods for storing and retrieving boolean value in NDK C++

MainActivity.java

public class MainActivity extends Activity {
    // Used to load the 'native-lib' library on application startup.
    static {
        System.loadLibrary("Native");
    }
    private Activity activity;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        activity=MainActivity.this;
        setStoreBoolValues(activity, true);

        if (getStoreValues(activity))
            Log.e("Store Value", " ** true **");
        else
            Log.e("Store Value", " ** false **");
    }

    public native boolean getStoreValues(Activity activity);
    public native void setStoreBoolValues(Activity activity, boolean flag);
}

Declared Native Method in MainActivity and also called my.so file

NativeClass.Cpp

jobject mainClass;

jstring spname(JNIEnv *env) {
    return env->NewStringUTF("sharedstore");
}

jstring objectname(JNIEnv *env) {
    return env->NewStringUTF("boolvaluestore");
}

extern "C"
JNIEXPORT jboolean JNICALL
Java_com_ebizzinfotech_amjad_contentresolverproj_MainActivity_getStoreValues(JNIEnv *env,
                                                                             jobject instance,
                                                                             jobject activity) {

    jclass spcls = env->FindClass("android/content/SharedPreferences");
    jclass contextcls = env->FindClass("android/content/Context");
    mainClass = env->NewGlobalRef(activity);
    jmethodID mid = env->GetMethodID(contextcls, "getSharedPreferences",
                                     "(Ljava/lang/String;I)Landroid/content/SharedPreferences;");
    jmethodID midbool = env->GetMethodID(spcls, "getBoolean",
                                         "(Ljava/lang/String;Z)Z");
    jobject jobjectshared = env->CallObjectMethod(mainClass, mid, spname(env), 0);
//    jobject jobjectsharededit  = env->CallObjectMethod(jobjectshared,midedit);
    jboolean jboolean1 = env->CallBooleanMethod(jobjectshared, midbool,objectname(env), JNI_FALSE);

    env->DeleteLocalRef(spcls);
    env->DeleteLocalRef(contextcls);
    return jboolean1;
}


extern "C"
JNIEXPORT void JNICALL
Java_com_ebizzinfotech_amjad_contentresolverproj_MainActivity_setStoreBoolValues(JNIEnv *env,
                                                                                 jobject instance,
                                                                                 jobject activity,
                                                                                 jboolean flag) {
    jclass spcls = env->FindClass("android/content/SharedPreferences");
    jclass speditorcls = env->FindClass("android/content/SharedPreferences$Editor");
    jclass contextcls = env->FindClass("android/content/Context");

    mainClass = env->NewGlobalRef(activity);

    jmethodID mid = env->GetMethodID(contextcls, "getSharedPreferences",
                                     "(Ljava/lang/String;I)Landroid/content/SharedPreferences;");

    jmethodID midedit = env->GetMethodID(spcls, "edit",
                                         "()Landroid/content/SharedPreferences$Editor;");

    jmethodID midputbool = env->GetMethodID(speditorcls, "putBoolean",
                                            "(Ljava/lang/String;Z)Landroid/content/SharedPreferences$Editor;");

    jmethodID midapply = env->GetMethodID(speditorcls, "apply",
                                          "()V");
    jobject jobjectshared = env->CallObjectMethod(mainClass, mid,spname(env), 0);
    jobject jobjectsharededit = env->CallObjectMethod(jobjectshared, midedit);
    env->CallVoidMethod(env->CallObjectMethod(jobjectsharededit, midputbool, objectname(env), flag),
                        midapply);
    env->DeleteLocalRef(spcls);
    env->DeleteLocalRef(contextcls);
    env->DeleteLocalRef(speditorcls);
}

Answer 4

I have created two.cpp classes: SharedPreferences for reading and SharedPreferences_Editor for writing They do not need any special java code and you should be able to include them in your project easily by copy-paste.

//
// Created by Constantin on 24.10.2017.
//

#ifndef FPV_VR_HELPER_SHARED_PREFERENCES_HPP
#define FPV_VR_HELPER_SHARED_PREFERENCES_HPP

#include <jni.h>
#include <android/log.h>
#include <string>

///Example reading values
///SharedPreferences sharedPref(env,context,"pref_telemetry");
///T_Protocol=sharedPref.getInt(IDT::T_Protocol);
///Example writing values
///SharedPreferences_Editor editor=sharedPref.edit();
///editor.putString("MY_KEY","HELLO");
///editor.commit();

class SharedPreferences_Editor{
public:
    SharedPreferences_Editor(JNIEnv* env,const jobject joSharedPreferences_Edit):env(env),joSharedPreferences_Edit(joSharedPreferences_Edit){
        //find the methods for putting values into Shared preferences via the editor
        jclass jcSharedPreferences_Editor = env->GetObjectClass(joSharedPreferences_Edit);
        jmPutBoolean=env->GetMethodID(jcSharedPreferences_Editor,"putBoolean","(Ljava/lang/String;Z)Landroid/content/SharedPreferences$Editor;");
        jmPutInt=env->GetMethodID(jcSharedPreferences_Editor,"putInt","(Ljava/lang/String;I)Landroid/content/SharedPreferences$Editor;");
        jmPutString=env->GetMethodID(jcSharedPreferences_Editor,"putString","(Ljava/lang/String;Ljava/lang/String;)Landroid/content/SharedPreferences$Editor;");
        jmCommit=env->GetMethodID(jcSharedPreferences_Editor,"commit","()Z");
    }
    //return itself for method chaining
    const SharedPreferences_Editor& putBoolean(const char* key,const bool value)const{
        env->CallObjectMethod(joSharedPreferences_Edit,jmPutBoolean,env->NewStringUTF(key),(jboolean)value);
        return *this;
    }
    const SharedPreferences_Editor& putInt(const char* key,const int value)const{
        env->CallObjectMethod(joSharedPreferences_Edit,jmPutInt,env->NewStringUTF(key),(jint)value);
        return *this;
    }
    const SharedPreferences_Editor& putString(const char* key,const char* value)const{
        env->CallObjectMethod(joSharedPreferences_Edit,jmPutString,env->NewStringUTF(key),env->NewStringUTF(value));
        return *this;
    }
    bool commit()const{
        return (bool)env->CallBooleanMethod(joSharedPreferences_Edit,jmCommit);
    }
private:
    JNIEnv* env;
    jobject joSharedPreferences_Edit;
    jmethodID jmPutBoolean;
    jmethodID jmPutInt;
    jmethodID jmPutString;
    jmethodID jmCommit;
};


class SharedPreferences {
public:
    SharedPreferences(SharedPreferences const &) = delete;
    void operator=(SharedPreferences const &)= delete;
public:
    //Note: Per default, this doesn't keep the reference to the sharedPreferences java object alive
    //longer than the lifetime of the JNIEnv.
    //With keepReference=true the joSharedPreferences is kept 'alive' and you can still use the class after the original JNIEnv* has become invalid -
    //but make sure to refresh the JNIEnv* object with a new valid reference via replaceJNI()
    SharedPreferences(JNIEnv *env, jobject androidContext,const char* name,const bool keepReference=false){
        this->env=env;
        //Find the 2 java classes we need to make calls with
        jclass jcContext = env->FindClass("android/content/Context");
        jclass jcSharedPreferences = env->FindClass("android/content/SharedPreferences");
        //jclass jcSharedPreferences_Editor=env->FindClass("android/content/SharedPreferences$Editor");
        if(jcContext==nullptr || jcSharedPreferences== nullptr){
            __android_log_print(ANDROID_LOG_DEBUG, "SharedPreferences","Cannot find classes");
        }
        //find the 3 functions we need to get values from an SharedPreferences instance
        jmGetBoolean=env->GetMethodID(jcSharedPreferences,"getBoolean","(Ljava/lang/String;Z)Z");
        jmGetInt=env->GetMethodID(jcSharedPreferences,"getInt","(Ljava/lang/String;I)I");
        jmGetFloat=env->GetMethodID(jcSharedPreferences,"getFloat","(Ljava/lang/String;F)F");
        jmGetString=env->GetMethodID(jcSharedPreferences,"getString","(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;");
        //find the 1 function we need to create the SharedPreferences.Editor object
        jmEdit=env->GetMethodID(jcSharedPreferences,"edit", "()Landroid/content/SharedPreferences$Editor;");
        //create a instance of SharedPreferences and store it in @joSharedPreferences
        jmethodID jmGetSharedPreferences=env->GetMethodID(jcContext,"getSharedPreferences","(Ljava/lang/String;I)Landroid/content/SharedPreferences;");
        joSharedPreferences=env->CallObjectMethod(androidContext,jmGetSharedPreferences,env->NewStringUTF(name),MODE_PRIVATE);
        //jmEdit_commit=env->GetMethodID(jcSharedPreferences_Editor,"putString","(Ljava/lang/String;Ljava/lang/String;)Landroid/content/SharedPreferences$Editor;");
        if(keepReference){
            joSharedPreferences=env->NewWeakGlobalRef(joSharedPreferences);
        }
    }
    void replaceJNI(JNIEnv* newEnv){
        env=newEnv;
    }
private:
    JNIEnv* env;
    jobject joSharedPreferences;
    jmethodID jmGetBoolean;
    jmethodID jmGetInt;
    jmethodID jmGetFloat;
    jmethodID jmGetString;
    jmethodID jmEdit;
public:
    bool getBoolean(const char* id,bool defaultValue=false)const{
        return (bool)(env->CallBooleanMethod(joSharedPreferences,jmGetBoolean,env->NewStringUTF(id),(jboolean)defaultValue));
    }
    int getInt(const char* id,int defaultValue=0)const{
        return (int)(env->CallIntMethod(joSharedPreferences,jmGetInt,env->NewStringUTF(id),(jint)defaultValue));
    }
    float getFloat(const char* id,float defaultValue=0.0f)const{
        return (float)(env->CallFloatMethod(joSharedPreferences,jmGetFloat,env->NewStringUTF(id),(jfloat)defaultValue));
    }
    std::string getString(const char* id,const char* defaultValue="")const{
        auto value=(jstring)(env->CallObjectMethod(joSharedPreferences,jmGetString,env->NewStringUTF(id),env->NewStringUTF(defaultValue)));
        const char* valueP = env->GetStringUTFChars(value, nullptr);
        const std::string ret=std::string(valueP);
        env->ReleaseStringUTFChars(value,valueP);
        //__android_log_print(ANDROID_LOG_DEBUG, "SharedPreferences","%s",ret.c_str());
        return ret;
    }
    SharedPreferences_Editor edit()const{
        //create a instance of SharedPreferences.Editor and store it in @joSharedPreferences_Edit
        jobject joSharedPreferences_Edit=env->CallObjectMethod(joSharedPreferences,jmEdit);
        SharedPreferences_Editor editor(env,joSharedPreferences_Edit);
        return editor;
    }
private:
    static constexpr const int  MODE_PRIVATE = 0; //taken directly from java, assuming this value stays constant in java
};


#endif //FPV_VR_HELPER_SHARED_PREFERENCES_HPP