Android Development NDK - Beginner's guide

The Java Native Interface (JNI) allows you to call native functions from Java code, and vice versa. This example shows how to load and call a native function via JNI, it does not go into accessing Java methods and fields from native code using JNI functions.

The method signatures in JNI C files contain the package name and class name as well. So you cannot call the same function from different activities/classes.

eg.

JNIEXPORT float JNICALL

Java_packageName_methodName(JNIEnv *env, jobject thiz)

{ }

This JNI method 'getPageWidth() 'can be called only from

com.taf.ndk.methodName

Accessing Object's Variables and Calling Back Methods

5.1 Accessing Object's Instance Variables

JNI Program - TestJNIInstanceVariable.java

public class TestJNIInstanceVariable {

   static {

      System.loadLibrary("myjni"); // myjni.dll (Windows) or libmyjni.so (Unixes)

   }

   // Instance variables

   private int number = 88;

   private String message = "Hello from Java";

   // Declare a native method that modifies the instance variables

   private native void modifyInstanceVariable();

   // Test Driver   

   public static void main(String args[]) {

      TestJNIInstanceVariable test = new TestJNIInstanceVariable();

      test.modifyInstanceVariable();

      System.out.println("In Java, int is " + test.number);

      System.out.println("In Java, String is " + test.message);

   }

}

The class contains two private instance variables: a primitive int called number and a String called message. It also declares a native method, which could modify the contents of the instance variables.

C Implementation - TestJNIInstanceVariable.c

#include <jni.h>

#include <stdio.h>

#include "TestJNIInstanceVariable.h"

JNIEXPORT void JNICALL Java_TestJNIInstanceVariable_modifyInstanceVariable

          (JNIEnv *env, jobject thisObj) {

   // Get a reference to this object's class

   jclass thisClass = (*env)->GetObjectClass(env, thisObj);

   // int

   // Get the Field ID of the instance variables "number"

   jfieldID fidNumber = (*env)->GetFieldID(env, thisClass, "number", "I");

   if (NULL == fidNumber) return;

   // Get the int given the Field ID

   jint number = (*env)->GetIntField(env, thisObj, fidNumber);

   printf("In C, the int is %d\n", number);

   // Change the variable

   number = 99;

   (*env)->SetIntField(env, thisObj, fidNumber, number);

   // Get the Field ID of the instance variables "message"

   jfieldID fidMessage = (*env)->GetFieldID(env, thisClass, "message", "Ljava/lang/String;");

   if (NULL == fidMessage) return;

   // String

   // Get the object given the Field ID

   jstring message = (*env)->GetObjectField(env, thisObj, fidMessage);

   // Create a C-string with the JNI String

   const char *cStr = (*env)->GetStringUTFChars(env, message, NULL);

   if (NULL == cStr) return;

   printf("In C, the string is %s\n", cStr);

   (*env)->ReleaseStringUTFChars(env, message, cStr);

   // Create a new C-string and assign to the JNI string

   message = (*env)->NewStringUTF(env, "Hello from C");

   if (NULL == message) return;

   // modify the instance variables

   (*env)->SetObjectField(env, thisObj, fidMessage, message);

}

To access the instance variable of an object:

1. Get a reference to this object's class via GetObjectClass().

2. Get the Field ID of the instance variable to be accessed via GetFieldID() from the class reference. You need to provide the variable name and its field descriptor (or signature). For a Java class, the field descriptor is in the form of "L<fully-qualified-name>;", with dot replaced by forward slash (/), e.g.,, the class descriptor for String is "Ljava/lang/String;". For primitives, use "I" for int, "B" for byte, "S" for short, "J" for long, "F" for float, "D" for double, "C" for char, and "Z" for boolean. For arrays, include a prefix "[", e.g., "[Ljava/lang/Object;" for an array of Object; "[I" for an array of int.

3. Based on the Field ID, retrieve the instance variable via GetObjectField() or Get<primitive-type>Field() function.

4. To update the instance variable, use the SetObjectField() or Set<primitive-type>Field() function, providing the Field ID.

The JNI functions for accessing instance variable are:

jclass GetObjectClass(JNIEnv *env, jobject obj);

   // Returns the class of an object.

jfieldID GetFieldID(JNIEnv *env, jclass cls, const char *name, const char *sig);

  // Returns the field ID for an instance variable of a class.

NativeType Get<type>Field(JNIEnv *env, jobject obj, jfieldID fieldID);

void Set<type>Field(JNIEnv *env, jobject obj, jfieldID fieldID, NativeType value);

  // Get/Set the value of an instance variable of an object

  // <type> includes each of the eight primitive types plus Object.

FFmpeg is the leading multimedia framework, able to decode, encode, transcode, mux, demux, stream, filter and play pretty much anything that humans and machines have created. It supports the most obscure ancient formats up to the cutting edge. No matter if they were designed by some standards committee, the community or a corporation. It is also highly portable: FFmpeg compiles, runs, and passes our testing infrastructure FATE across Linux, Mac OS X, Microsoft Windows, the BSDs, Solaris, etc. under a wide variety of build environments, machine architectures, and configurations.

It contains libavcodec, libavutil, libavformat, libavfilter, libavdevice, libswscale and libswresample which can be used by applications. As well as ffmpeg, ffplay and ffprobe which can be used by end users for transcoding and playing.

EXAMPLES

  Cut video by time

    ffmpeg -i video.mp4 -ss 00:01:11 -t 00:00:08 -async 1 out.mp4

          # -t is duration, NOT endtime

  Converting a video to images

    mkdir frames

    ffmpeg -i video.mp4 'frames/frame-%03d.jpg'

          # change %03 to %0N for N digits in the filename

  Converting a video to a .gif

    mkdir frames

    ffmpeg -i video.mp4 -r 20 'frames/frame-%03d.jpg'

          # -r [N] for [N] fps

    convert -resize 768x576 -delay 5 -loop 0 frames/*.jpg video.gif

          # -resize is useful for reducing filesize

          # -delay [N] with [N] = 100/fps

          # -loop 0 to loop forever

  Converting frames to video

    ffmpeg -framerate 30 -pattern_type glob -i 'frames/*.jpg' -c:v libx264 -pix_fmt yuv420p out.mp4

  Scale a video

    ffmpeg -i input.avi -vf scale=852:480 output.avi

          # scale=852:-1   to preserve the aspect ratio

  Rotate a video

    ffmpeg -i in.mov -vf "transpose=1" out.mov

          # transpose:

          #   0 = 90CounterCLockwise and Vertical Flip (default)

          #   1 = 90Clockwise

          #   2 = 90CounterClockwise

          #   3 = 90Clockwise and Vertical Flip

  Speed up / Slow down a video

    ffmpeg -i input.mkv -filter:v "setpts=0.5*PTS" -an output.mkv

          #      this multiplies all timestamps by 0.5, making the video

          #      2x as fast, without interpolation.

          #      frames are skipped in order to keep framerate.

          # -an  disables the audio stream

NOTES

  Experimental codecs

    # Getting this kind of error?

    #   "The encoder 'aac' is experimental but experimental codecs

    #    are not enabled, add '-strict -2' if you want to use it."

    # -strict -2 needs to be added right before the last parameter

    ffmpeg -i infile -strict -2 outfile

Oboe is a C++ library which makes it easy to build high-performance audio apps on Android. Apps communicate with Oboe by reading and writing data to streams.