• Licence
• Introduction
• Java interface
• Native library implementation
  • Headers and global variables
  • Calling Java functions from Java and native threads
  • Implementation of other native functions
  • The JNI_OnLoad() function implementation
• Building the native library
• Using native functions in Java code
• Unresolved issues and bugs

[*]Licence

Copyright (C) 2009 Jurij Smakov <jurij@wooyd.org>

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

[*]Introduction

This document has been generated from source using noweb, a literate programming tool. The JNIExample.nw is the source in noweb format. It can be used to generate the document output in a variety of formats (for example, PDF), as well as generate the JNI example source code.

The complete Android project, including the source code generated from JNIExample.nw is available for download. So, if you are impatient, just grab it and check out the "Building the native library" section [->], which describes prerequisites for the build and the build procedure itself.

This document is not a replacement for other general JNI documentation. If you are not familiar with JNI, you may want to have a look at the following resources:

• Sun's Java Native Interface guide
• Java Native Interface: Programmer's Guide and Specification

• JNI in Android
• How to add a new module to Android

If you notice any errors or omissions (there are a couple of known bugs and unresolved issues [->]), or have a suggestion on how to improve this document, feel free to contact me using the email address mentioned above.

[*]Java interface

The Java functions which are backed by the corresponding native functions are declared in a usual way, adding a native qualifier. We also want to demonstrate how we could do the callbacks, i.e. calling the Java code from native code. That leads to the following high-level view of our interface class:

<JNIExampleInterface.java>=
package org.wooyd.android.JNIExample;

import android.os.Handler;
import android.os.Bundle;
import android.os.Message;
import org.wooyd.android.JNIExample.Data;

public class JNIExampleInterface {
    static Handler h;
    <Example constructors>
    <Example native functions>
    <Example callback>
}

One valid question about this definition is why we need a Handler class attribute. It turns out that it will come in handy in situations, when the native library wants to pass some information to the Java process through a callback. If the callback will be called by a native thread (for extended discussion see "Calling Java functions" section [->]), and then will try to modify the application's user interface (UI) in any way, an exception will be thrown, as Android only allows the thread which created the UI (the UI thread) to modify it. To overcome this problem we are going to use the message-passing interface provided by Handler to dispatch the data received by a callback to the UI thread, and allow it to do the UI modifications. In order for this to work, we are going to accept a Handler instance as an argument for non-trivial constructor (reasons for keeping trivial one will become apparent later), and save it in a class attribute, and that's pretty much the only task for the constructor:

<Example constructors>= (<-U)
    public JNIExampleInterface() {}
    public JNIExampleInterface(Handler h) {
        this.h = h;
    }

To illustrate various argument-passing techniques, we define three native functions:

• callVoid(): takes no arguments and returns nothing;
• getNewData(): takes two arguments and constructs a new class instance using them;
• getDataString(): extracts a value from an object, which is passed as an argument.

<Example native functions>= (<-U)
    public static native void callVoid();
    public static native Data getNewData(int i, String s);
    public static native String getDataString(Data d);

The callback will receive a string as an argument, and dispatch it to the Handler instance recorded in the constructor, after wrapping it in a Bundle:

<Example callback>= (<-U)
    public static void callBack(String s) {
        Bundle b = new Bundle();
        b.putString("callback_string", s);
        Message m = Message.obtain();
        m.setData(b);
        m.setTarget(h);
        m.sendToTarget();
    }

We also need a definition of a dummy Data class, used purely for illustrative purposes:

<Data.java>=
package org.wooyd.android.JNIExample;

public class Data {
    public int i;
    public String s;
    public Data() {}
    public Data(int i, String s) {
        this.i = i;
        this.s = s;
    }
}

After the source files Data.java and JNIExampleInterface.java are compiled, we can generate the JNI header file, containing the prototypes of the native functions, corresponding to their Java counterparts:

$ javac -classpath /path/to/sdk/android.jar \
        org/wooyd/android/JNIExample/*.java
$ javah -classpath . org.wooyd.android.JNIExample.JNIExampleInterface

Native library implementation

<JNIExample.cpp>=
<JNI includes>
<Miscellaneous includes>
<Global variables>
#ifdef __cplusplus
extern "C" {
#endif
<callVoid implementation>
<getNewData implementation>
<getDataString implementation>
<initClassHelper implementation>
<JNIOnLoad implementation>
#ifdef __cplusplus
}
#endif

Headers and global variables

<JNI includes>= (<-U)
#include <jni.h>
#include <JNIHelp.h>
#include <android_runtime/AndroidRuntime.h>

Various other things which will come in handy:

<Miscellaneous includes>= (<-U)
#include <string.h>
#include <unistd.h>
#include <pthread.h>

It is useful to have some global variables to cache things which we know will not change during the lifetime of our program, and can be safely used across multiple threads. One of such things is the JVM handle. We can retrieve it every time it's needed (for example, using android::AndroidRuntime::getJavaVM() function), but as it does not change, it's better to cache it.

We can also use global variables to cache the references to required classes. As described below, it is not always easy to do class resolution in native code, especially when it is done from native threads (see "Calling Java functions" section [->] for details). Here we are just providing the global variables to hold instances of Data and JNIExampleInterface class objects, as well as defining some constant strings which will come in handy:

<Global variables>= (<-U)
static JavaVM *gJavaVM;
static jobject gInterfaceObject, gDataObject;
const char *kInterfacePath = "org/wooyd/android/JNIExample/JNIExampleInterface";
const char *kDataPath = "org/wooyd/android/JNIExample/Data";

Defines gJavaVM, jobject, kDataPath, kInterfacePath (links are to index).

[*]Calling Java functions from Java and native threads

At this point it is important to recognize that there are two distinct possibilities here: the Java function may be called either from a thread which originated in Java or from a native thread, which has been started in the native code, and of which JVM has no knowledge of. In the former case the call may be performed directly, in the latter we must first attach the native thread to the JVM. That requires an additional layer, a native callback handler, which will do the right thing in either case. We will also need a function to create the native thread, so structurally the implementation will look like this:

<callVoid implementation>= (<-U)
<Callback handler>
<Thread start function>
<callVoid function>

Native callback handler gets the JNI environment (attaching the native thread if necessary), uses a cached reference to the gInterfaceObject to get to JNIExampleInterface class, obtains callBack() method reference, and calls it:

<Callback handler>= (<-U)
static void callback_handler(char *s) {
    int status;
    JNIEnv *env;
    bool isAttached = false;
   
    status = gJavaVM->GetEnv((void **) &env, JNI_VERSION_1_4);
    if(status < 0) {
        LOGE("callback_handler: failed to get JNI environment, "
             "assuming native thread");
        status = gJavaVM->AttachCurrentThread(&env, NULL);
        if(status < 0) {
            LOGE("callback_handler: failed to attach "
                 "current thread");
            return;
        }
        isAttached = true;
    }
    /* Construct a Java string */
    jstring js = env->NewStringUTF(s);
    jclass interfaceClass = env->GetObjectClass(gInterfaceObject);
    if(!interfaceClass) {
        LOGE("callback_handler: failed to get class reference");
        if(isAttached) gJavaVM->DetachCurrentThread();
        return;
    }
    /* Find the callBack method ID */
    jmethodID method = env->GetStaticMethodID(
        interfaceClass, "callBack", "(Ljava/lang/String;)V");
    if(!method) {
        LOGE("callback_handler: failed to get method ID");
        if(isAttached) gJavaVM->DetachCurrentThread();
        return;
    }
    env->CallStaticVoidMethod(interfaceClass, method, js);
    if(isAttached) gJavaVM->DetachCurrentThread();
}

Defines callback_handler (links are to index).

A few comments are in order:

• The JNI environment, returned by the JNI GetEnv() function is unique for each thread, so must be retrieved every time we enter the function. The JavaVM pointer, on the other hand, is per-program, so can be cached (you will see it done in the JNI_OnLoad() function), and safely used across threads.

• When we attach a native thread, the associated Java environment comes with a bootstrap class loader. That means that even if we would try to get a class reference in the function (the normal way to do it would be to use FindClass() JNI function), it would trigger an exception. Because of that we use a cached copy of JNIExampleInterface object to get a class reference (amusingly, we cannot cache the reference to the class itself, as any attempt to use it triggers an exception from JVM, who thinks that such reference should not be visible to native code). This caching is also done in JNI_OnLoad(), which might be the only function called by Android Java implementation with a functional class loader.

• In order to retrieve the method ID of the callBack() method, we need to specify its name and JNI signature. In this case the signature indicates that the function takes a java.lang.String object as an argument, and returns nothing (i.e. has return type void). Consult JNI documentation for more information on function signatures, one useful tip is that you can use javap utility to look up the function signatures of non-native functions (for native functions the signature information is already included as comments into the header, generated by javah).

• Someone more paranoid than me could use locking to avoid race conditions associated with setting and checking of the isAttached variable.

In order to test calling from native threads, we will also need a function which is started in a separate thread. Its only role is to call the callback handler:

<Thread start function>= (<-U)
void *native_thread_start(void *arg) {
    sleep(1);
    callback_handler((char *) "Called from native thread");
}

Defines native_thread_start (links are to index).

We now have all necessary pieces to implement the native counterpart of the callVoid() function:

<callVoid function>= (<-U)
/*
 * Class:     org_wooyd_android_JNIExample_JNIExampleInterface
 * Method:    callVoid
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_org_wooyd_android_JNIExample_JNIExampleInterface_callVoid
  (JNIEnv *env, jclass cls) {
    pthread_t native_thread;

    callback_handler((char *) "Called from Java thread");
    if(pthread_create(&native_thread, NULL, native_thread_start, NULL)) {
        LOGE("callVoid: failed to create a native thread");
    }
}

Defines JNICALL (links are to index).

Implementation of other native functions

<getNewData implementation>= (<-U)
/*
 * Class:     org_wooyd_android_JNIExample_JNIExampleInterface
 * Method:    getNewData
 * Signature: (ILjava/lang/String;)Lorg/wooyd/android/JNIExample/Data;
 */
JNIEXPORT jobject JNICALL Java_org_wooyd_android_JNIExample_JNIExampleInterface_getNewData
  (JNIEnv *env, jclass cls, jint i, jstring s) {
    jclass dataClass = env->GetObjectClass(gDataObject);
    if(!dataClass) {
        LOGE("getNewData: failed to get class reference");
        return NULL;
    }
    jmethodID dataConstructor = env->GetMethodID(
        dataClass, "<init>", "(ILjava/lang/String;)V");
    if(!dataConstructor) {
        LOGE("getNewData: failed to get method ID");
        return NULL;
    }
    jobject dataObject = env->NewObject(dataClass, dataConstructor, i, s);
    if(!dataObject) {
        LOGE("getNewData: failed to create an object");
        return NULL;
    }
    return dataObject;
}

Defines jobject (links are to index).

The getDataString() function illustrates how a value stored in an object's attribute can be retrieved in a native function.

<getDataString implementation>= (<-U)
/*
 * Class:     org_wooyd_android_JNIExample_JNIExampleInterface
 * Method:    getDataString
 * Signature: (Lorg/wooyd/android/JNIExample/Data;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_org_wooyd_android_JNIExample_JNIExampleInterface_getDataString
  (JNIEnv *env, jclass cls, jobject dataObject) {
    jclass dataClass = env->GetObjectClass(gDataObject);
    if(!dataClass) {
        LOGE("getDataString: failed to get class reference");
        return NULL;
    }
    jfieldID dataStringField = env->GetFieldID(
        dataClass, "s", "Ljava/lang/String;");
    if(!dataStringField) {
        LOGE("getDataString: failed to get field ID");
        return NULL;
    }
    jstring dataStringValue = (jstring) env->GetObjectField(
        dataObject, dataStringField);
    return dataStringValue;
}

Defines jstring (links are to index).

The JNI_OnLoad() function implementation

<JNIOnLoad implementation>= (<-U)
jint JNI_OnLoad(JavaVM* vm, void* reserved)
{
    JNIEnv *env;
    gJavaVM = vm;
    LOGI("JNI_OnLoad called");
    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
        LOGE("Failed to get the environment using GetEnv()");
        return -1;
    }
    <Class instance caching>
    <Native function registration>
    return JNI_VERSION_1_4;
}

We need some way to cache a reference to a class, because native threads do not have access to a functional classloader. As explained above, we can't cache the class references themselves, as it makes JVM unhappy. Instead we cache instances of these classes, so that we can later retrieve class references using GetObjectClass() JNI function. One thing to remember is that these objects must be protected from garbage-collecting using NewGlobalRef(), as that guarantees that they will remain available to different threads during JVM lifetime. Creating the instances and storing them in the global variables is the job for the initClassHelper() function:

<initClassHelper implementation>= (<-U)
void initClassHelper(JNIEnv *env, const char *path, jobject *objptr) {
    jclass cls = env->FindClass(path);
    if(!cls) {
        LOGE("initClassHelper: failed to get %s class reference", path);
        return;
    }
    jmethodID constr = env->GetMethodID(cls, "<init>", "()V");
    if(!constr) {
        LOGE("initClassHelper: failed to get %s constructor", path);
        return;
    }
    jobject obj = env->NewObject(cls, constr);
    if(!obj) {
        LOGE("initClassHelper: failed to create a %s object", path);
        return;
    }
    (*objptr) = env->NewGlobalRef(obj);

}

Defines initClassHelper (links are to index).

With this function defined, class instance caching is trivial:

<Class instance caching>= (<-U)
    initClassHelper(env, kInterfacePath, &gInterfaceObject);
    initClassHelper(env, kDataPath, &gDataObject);

In order to register the native functions, we create an array of JNINativeMethod structures, which contain function names, signatures (they can be simply copied from the comments, generated by javah), and pointers to the implementing functions. This array is then passed to Android's registerNativeMethods() function:

<Native function registration>= (<-U)
    JNINativeMethod methods[] = {
        {
            "callVoid",
            "()V",
            (void *) Java_org_wooyd_android_JNIExample_JNIExampleInterface_callVoid
        },
        {
            "getNewData",
            "(ILjava/lang/String;)Lorg/wooyd/android/JNIExample/Data;",
            (void *) Java_org_wooyd_android_JNIExample_JNIExampleInterface_getNewData
        },
        {
            "getDataString",
            "(Lorg/wooyd/android/JNIExample/Data;)Ljava/lang/String;",
            (void *) Java_org_wooyd_android_JNIExample_JNIExampleInterface_getDataString
        }
    };
    if(android::AndroidRuntime::registerNativeMethods(
        env, kInterfacePath, methods, NELEM(methods)) != JNI_OK) {
        LOGE("Failed to register native methods");
        return -1;
    }

[*]Building the native library

For an example of CXXFLAGS and LDFLAGS you need to use to create a shared library with Android toolchain, check out the Makefile, included in the example project tarball. They are derived from build/core/combo/linux-arm.mk in Android source.

You will probably want to build the entire example project, so you will need a copy of the SDK as well. This code has been tested to build with Android's 1.1 SDK and run on the currently released version of the phone. Once you downloaded the SDK and the example tarball and unpacked them, you can build the project using the command

ANDROID_DIR=/path/to/android/source SDK_DIR=/path/to/sdk make

Using native functions in Java code

<JNIExample.java>=
package org.wooyd.android.JNIExample;
<Imports>
public class JNIExample extends Activity
{
    TextView callVoidText, getNewDataText, getDataStringText;
    Button callVoidButton, getNewDataButton, getDataStringButton;
    Handler callbackHandler;
    JNIExampleInterface jniInterface;

    @Override
    public void onCreate(Bundle savedInstanceState) 
    {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.main);
         <Load JNI library>
         <callVoid demo>
         <getNewData demo>
         <getDataString demo>
    }
}

Imports needed to draw the UI and display it to the user:

<Imports>= (<-U) [D->]
    import android.app.Activity;
    import android.view.View;
    import android.widget.Button;
    import android.widget.TextView;

Imports needed to enable communication between the Java callback and the UI thread:

<Imports>+= (<-U) [<-D->]
    import android.os.Bundle;
    import android.os.Handler;
    import android.os.Message;

Imports for manipulation with the native library:

<Imports>+= (<-U) [<-D->]
    import java.util.zip.*;
    import java.io.InputStream;
    import java.io.OutputStream;
    import java.io.FileOutputStream;
    import java.io.File;

We will also need access to our JNI interface class and toy Data class:

<Imports>+= (<-U) [<-D->]
    import org.wooyd.android.JNIExample.JNIExampleInterface;
    import org.wooyd.android.JNIExample.Data;

Logging utilities will also come in handy:

<Imports>+= (<-U) [<-D]
    import android.util.Log;

At this time the only officialy supported way to create an Android application is by using the Java API. That means, that no facilities are provided to easily build and package shared libraries, and automatically load them on application startup. One possible way to include the library into the application package (file with extension .apk) is to place it into the assets subdirectory of the Android project, created with activitycreator. During the package build it will be automatically included into the APK package, however we still will have to load it by hand when our application starts up. Luckily, the location where APK is installed is known, and APK is simply a ZIP archive, so we can extract the library file from Java and copy it into the application directory, allowing us to load it:

<Load JNI library>= (<-U)
    try {
        String cls = "org.wooyd.android.JNIExample";
        String lib = "libjniexample.so";
        String apkLocation = "/data/app/" + cls + ".apk";
        String libLocation = "/data/data/" + cls + "/" + lib;
        ZipFile zip = new ZipFile(apkLocation);
        ZipEntry zipen = zip.getEntry("assets/" + lib);
        InputStream is = zip.getInputStream(zipen);
        OutputStream os = new FileOutputStream(libLocation);
        byte[] buf = new byte[8092];
        int n;
        while ((n = is.read(buf)) > 0) os.write(buf, 0, n);
        os.close();
        is.close();
        System.load(libLocation);
    } catch (Exception ex) {
         Log.e("JNIExample", "failed to install native library: " + ex);
    }

The rest simply demonstrates the functionality, provided by the native library, by calling the native functions and displaying the results. For the callVoid() demo we need to initialize a handler first, and pass it to the JNI interface class, to enable us to receive callback messages:

<callVoid demo>= (<-U) [D->]
    callVoidText = (TextView) findViewById(R.id.callVoid_text);
    callbackHandler = new Handler() {
        public void handleMessage(Message msg) {
            Bundle b = msg.getData();
            callVoidText.setText(b.getString("callback_string"));
        }
    };
    jniInterface = new JNIExampleInterface(callbackHandler);

We also set up a button which will call callVoid() from the native library when pressed:

<callVoid demo>+= (<-U) [<-D]
    callVoidButton = (Button) findViewById(R.id.callVoid_button);
    callVoidButton.setOnClickListener(new Button.OnClickListener() {
        public void onClick(View v) {
            jniInterface.callVoid();
            
        } 
    });

For getNewData() we pass the parameters to the native function and expect to get the Data object back:

<getNewData demo>= (<-U)
    getNewDataText = (TextView) findViewById(R.id.getNewData_text);  
    getNewDataButton = (Button) findViewById(R.id.getNewData_button);
    getNewDataButton.setOnClickListener(new Button.OnClickListener() {
        public void onClick(View v) {
            Data d = jniInterface.getNewData(42, "foo");
            getNewDataText.setText(
                "getNewData(42, \"foo\") == Data(" + d.i + ", \"" + d.s + "\")");
        }
    });

And pretty much the same for getDataString():

<getDataString demo>= (<-U)
    getDataStringText = (TextView) findViewById(R.id.getDataString_text);  
    getDataStringButton = (Button) findViewById(R.id.getDataString_button);
    getDataStringButton.setOnClickListener(new Button.OnClickListener() {
        public void onClick(View v) {
            Data d = new Data(43, "bar");
            String s = jniInterface.getDataString(d);
            getDataStringText.setText(
                "getDataString(Data(43, \"bar\")) == \"" + s + "\"");
        }
    });

Try pushing the buttons and see whether it actually works!

[*]Unresolved issues and bugs

• <Callback handler>: U1, D2
• <callVoid demo>: U1, D2, D3
• <callVoid function>: U1, D2
• <callVoid implementation>: U1, D2
• <Class instance caching>: U1, D2
• <Data.java>: D1
• <Example callback>: U1, D2
• <Example constructors>: U1, D2
• <Example native functions>: U1, D2
• <getDataString demo>: U1, D2
• <getDataString implementation>: U1, D2
• <getNewData demo>: U1, D2
• <getNewData implementation>: U1, D2
• <Global variables>: U1, D2
• <Imports>: U1, D2, D3, D4, D5, D6
• <initClassHelper implementation>: U1, D2
• <JNI includes>: U1, D2
• <JNIExample.cpp>: D1
• <JNIExample.java>: D1
• <JNIExampleInterface.java>: D1
• <JNIOnLoad implementation>: U1, D2
• <Load JNI library>: U1, D2
• <Miscellaneous includes>: U1, D2
• <Native function registration>: U1, D2
• <Thread start function>: U1, D2

• callback_handler: D1, U2, U3
• gJavaVM: D1, U2, U3
• initClassHelper: D1, U2
• JNICALL: D1, U2, U3
• jobject: D1, D2, U3, U4
• jstring: U1, U2, D3
• kDataPath: D1, U2
• kInterfacePath: D1, U2, U3
• native_thread_start: D1, U2