Android如何通过JNi示例使用libjpeg-turbo库

Question

我成功地用ndk构建了libjpeg-turbo，这要感谢以下文章：libjpeg-turbo for android

我想在libjpeg-turbo的例子中获得一个像read_JPEG_file这样的本地函数，以便从Java代码中调用，将其用于Android应用程序。

有人能给我举个例子吗？如何使用libjpeg-turbo构建库为Java编写本机方法？

我可以通过

System.loadLibrary("libjpeg");

但是接下来呢？库没有任何可从Java调用的本地方法。

我试图根据JNI文档编写一个JNI c类，但没有成功。示例代码将很好地学习如何做到这一点。

编辑：

我创建了一个测试类NativeMethods：

package com.test.app;

public class NativeMethods {

    private String filename = null;

    static {
        System.loadLibrary("jpeg"); // this represents compiled libjpeg-turbo under ndk
    }

    public NativeMethods(String jpegFilename) {
        this.filename = jpegFilename;
    }

    public native int computeNumberOfDCTS(String filename);
}

然后使用javah生成本机方法的C头，结果是包含以下内容的com_test_app_NativeMethods.h文件：

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_test_app_NativeMethods */

#ifndef _Included_com_test_app_NativeMethods
#define _Included_com_test_app_NativeMethods
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_test_app_NativeMethods
 * Method:    computeNumberOfDCTS
 * Signature: (Ljava/lang/String;)I
 */
JNIEXPORT jint JNICALL Java_com_test_app_NativeMethods_computeNumberOfDCTS
  (JNIEnv *, jobject, jstring);

#ifdef __cplusplus
}
#endif
#endif

然后，我创建了名为JPEGProcessing.c的文件，将本机函数的c++实现如下所示：

#include "com_test_app_NativeMethods.h"

//that came from jpeglib example
struct my_error_mgr
{
    struct jpeg_error_mgr pub;
    jmp_buf setjmp_buffer;
};
typedef struct my_error_mgr * my_error_ptr;

//routine that will replace the standard error_exit method
static void
my_error_exit (j_common_ptr cinfo)
{
    /* cinfo->err really points to a my_error_mgr struct, so coerce pointer */
    my_error_ptr myerr = (my_error_ptr) cinfo->err;

    (*cinfo->err->output_message) (cinfo);

    /* Return control to the setjmp point */
    longjmp(myerr->setjmp_buffer, 1);
}

JNIEXPORT jint JNICALL Java_com_test_app_NativeMethods_computeNumberOfDCTS
    (JNIEnv *env, jobject obj, jstring javaString)
{
    jint toReturn = 0;
    // struct representing jpeg image
    struct jpeg_decompress_struct  cinfo;
    // struct representing error manager; defined above
    struct my_error_mgr jerr;

    const char *filename = (*env)->GetStringUTFChars(env, javaString, 0);

    FILE * infile;

    if ((infile = fopen(filename, "rb")) == NULL) {
        fprintf(stderr, "can't open %s\n", filename);
        return -1;
    }

    cinfo.err = jpeg_std_error(&jerr.pub);
    jerr.pub.error_exit = my_error_exit;

    if (setjmp(jerr.setjmp_buffer)) {
        jpeg_destroy_decompress(&cinfo);
        fclose(infile);
        return -1;
    }

    jpeg_create_decompress(&cinfo);

    jpeg_stdio_src(&cinfo, infile);

    (void) jpeg_read_header(&cinfo, TRUE);

    // declare virtual arrays for DCT coefficients
    jvirt_barray_ptr* coeffs_array;
    // read DCT coefficients from jpeg
    coeffs_array = jpeg_read_coefficients(&cinfo);

    // fill virtual arrays.
    // ci represents component color
    // this cycle prints all dct coefficient of the jpeg in 8x8 blocks
    for (int ci = 0; ci < 3; ci++)
    {
        JBLOCKARRAY buffer_one;
        JCOEFPTR blockptr_one;
        jpeg_component_info* compptr_one;
        compptr_one = cinfo.comp_info + ci;

        for (int by = 1; by < (compptr_one->height_in_blocks - 1); by++) // we don't want to use the edges of the images
        {
            buffer_one = (cinfo.mem->access_virt_barray)((j_common_ptr)&cinfo, coeffs_array[ci], by, (JDIMENSION)1, FALSE);

            for (int bx = 1; bx < (compptr_one->width_in_blocks - 1); bx++) // we don't want to use the edges of the images
            {
                blockptr_one = buffer_one[0][bx];

                for (int bi = 1; bi < 64; bi++) // we don't want to use AC
                {
                    toReturn++;
                }
            }
        }
    }

    jpeg_destroy_decompress(&cinfo);
    fclose(infile);

    return toReturn;
}

JPEGProcessing.c和com_test_app_NativeMethods.h文件位于project/jni/文件夹中。在同一个jni文件夹中，我创建了Android.mk文件，其中放置了以下行：

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)

LOCAL_MODULE    := JPEGProcessing
LOCAL_CFLAGS    := -Werror
LOCAL_SRC_FILES := JPEGProcessing.c

LOCAL_C_INCLUDES := $(LOCAL_PATH)
LOCAL_LDLIBS    := -lm -llog -landroid
LOCAL_STATIC_LIBRARIES := libjpeg
include $(BUILD_SHARED_LIBRARY)

在从我的项目目录运行ndk-build之后，出现了一些错误：

jni/JPEGProcessing.c:7:27: error: field 'pub' has incomplete type
jni/JPEGProcessing.c:8:5: error: unknown type name 'jmp_buf'
jni/JPEGProcessing.c:14:16: error: unknown type name 'j_common_ptr'
jni/JPEGProcessing.c: In function 'Java_com_test_app_NativeMethods_computeNumberOfDCTS':
jni/JPEGProcessing.c:30:36: error: storage size of 'cinfo' isn't known
jni/JPEGProcessing.c:36:5: error: unknown type name 'FILE'
jni/JPEGProcessing.c:38:17: error: assignment makes pointer from integer without a cast [-Werror]
jni/JPEGProcessing.c:38:45: error: 'NULL' undeclared (first use in this function)
...

我还是不明白。如何将代码与libjpeg-turbo函数相结合以获得工作库并在java中使用？

我读过NDK的说明和例子，但仍然不明白。

编辑：

NDK和JNI使用简单的本地方法工作得很好。作为测试，我使用了以下简单的方法，效果很好：

#include "com_test_app_NativeMethods.h"

JNIEXPORT jint JNICALL Java_com_test_app_NativeMethods_computeNumberOfDCTS
    (JNIEnv *env, jobject obj, jstring javaString)
{
    jint toReturn = 0;

    const char *filename = (*env)->GetStringUTFChars(env, javaString, 0);
    const char *test = "test";

    if ( filename == test )
    {
        toReturn++;
    }

    return toReturn;
}

我尝试了libjpeg-turbo的东西，如下所示：

#include "com_test_app_NativeMethods.h"
#include <stdio.h>
#include <setjmp.h>
#include <libjpeg-turbo/jpeglib.h>
#include <libjpeg-turbo/turbojpeg.h>
#include <libjpeg-turbo/jconfig.h>
#include <libjpeg-turbo/jmorecfg.h>
#include <libjpeg-turbo/jerror.h>

//that came from jpeglib example
struct my_error_mgr
{
    struct jpeg_error_mgr pub;
    jmp_buf setjmp_buffer;
};
typedef struct my_error_mgr * my_error_ptr;

//routine that will replace the standard error_exit method
static void
my_error_exit (j_common_ptr cinfo)
{
    /* cinfo->err really points to a my_error_mgr struct, so coerce pointer */
    my_error_ptr myerr = (my_error_ptr) cinfo->err;

    (*cinfo->err->output_message) (cinfo);

    /* Return control to the setjmp point */
    longjmp(myerr->setjmp_buffer, 1);
}

JNIEXPORT jint JNICALL Java_com_test_app_NativeMethods_computeNumberOfDCTS
    (JNIEnv *env, jobject obj, jstring javaString)
{
    jint toReturn = 0;
    // struct representing jpeg image
    struct jpeg_decompress_struct  cinfo;
    // struct representing error manager; defined above
    struct my_error_mgr jerr;

    const char *filename = (*env)->GetStringUTFChars(env, javaString, 0);

    FILE * infile;

    if ((infile = fopen(filename, "rb")) == NULL) {
        // cannot open file
        return -1;
    }

    cinfo.err = jpeg_std_error(&jerr.pub);
    jerr.pub.error_exit = my_error_exit;

    if (setjmp(jerr.setjmp_buffer)) {
        jpeg_destroy_decompress(&cinfo);
        fclose(infile);
        return -1;
    }

    jpeg_create_decompress(&cinfo);

    jpeg_destroy_decompress(&cinfo);
    fclose(infile);

    return toReturn;
}

我得到了错误.

Compile thumb  : JPEGProcessing <= JPEGProcessing.c
SharedLibrary  : libJPEGProcessing.so
/usr/android-ndk-r8c/toolchains/arm-linux-androideabi-4.6/prebuilt/darwin-x86/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: ./obj/local/armeabi/objs/JPEGProcessing/JPEGProcessing.o: in function Java_com_test_app_NativeMethods_computeNumberOfDCTS:jni/JPEGProcessing.c:50: error: undefined reference to 'jpeg_std_error'
/usr/android-ndk-r8c/toolchains/arm-linux-androideabi-4.6/prebuilt/darwin-x86/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: ./obj/local/armeabi/objs/JPEGProcessing/JPEGProcessing.o: in function Java_com_test_app_NativeMethods_computeNumberOfDCTS:jni/JPEGProcessing.c:59: error: undefined reference to 'jpeg_CreateDecompress'
/usr/android-ndk-r8c/toolchains/arm-linux-androideabi-4.6/prebuilt/darwin-x86/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: ./obj/local/armeabi/objs/JPEGProcessing/JPEGProcessing.o: in function Java_com_test_app_NativeMethods_computeNumberOfDCTS:jni/JPEGProcessing.c:61: error: undefined reference to 'jpeg_destroy_decompress'
/usr/android-ndk-r8c/toolchains/arm-linux-androideabi-4.6/prebuilt/darwin-x86/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld: ./obj/local/armeabi/objs/JPEGProcessing/JPEGProcessing.o: in function Java_com_test_app_NativeMethods_computeNumberOfDCTS:jni/JPEGProcessing.c:54: error: undefined reference to 'jpeg_destroy_decompress'
collect2: ld returned 1 exit status
make: *** [obj/local/armeabi/libJPEGProcessing.so] Error 1

那么，如何在ndk构建的libjpeg-turbo库中使用本机代码呢？

Answer 1

如果需要从本机代码执行所有这些操作，则可以在本机端为库创建自己的包装器。这些步骤大致如下：

• 在你打电话的班级里： System.loadLibrary("libjpeg")；

在Java端声明需要调用的本机方法，而不提供任何实现：

public static native readJPEGFile();

• 然后，您可以使用javah命令创建一个C文件，并在本机端实现您的本机funcions。在Java端调用本机函数时将调用该函数。请注意函数的完整路径和参数，这些参数将由javah命令自动生成，不应更改，否则将无法工作。结果应该是这样的： 导出"C“{ JNIEXPORT void Java_com_android_packagename_GL2JNILib_readJPEGFile(JNIEnv * env，jobject obj)；}；JNIEXPORT void Java_com_android_packagename_GL2JNILib_readJPEGFile(JNIEnv * env，jobject obj) { //调用库的函数}
• 之后，您需要在jni文件夹中创建一个包含包装器函数的共享库(如果不存在，则创建它)。您可以创建一个Android.mk文件并运行ndk来生成共享库(.so文件)。可能的Android.mk可能是这样的，但是您应该检查名称和依赖项： LOCAL_PATH := $(调用my-dir)包括$(CLEAR_VARS) LOCAL_MODULE := jpgturbo-包装器LOCAL_CFLAGS := -Werror LOCAL_SRC_FILES := jpgturboWrapper.cpp LOCAL_C_INCLUDES := (LOCAL_PATH) LOCAL_LDLIBS := -lm LOCAL_PATH jpegturbo #在这里编写项目所依赖的所有库，用包含$()的空格分隔。
• 现在您可以运行ndk，.so文件将生成并复制到它的对应文件夹中。

您可以先查看NDK文档，特别是它们提供的示例：http://developer.android.com/tools/sdk/ndk/index.html。

有时候，在github上进行快速搜索也是个好主意，看看是否有其他人在你之前就把工作放在了同一个库中，或者只是查看一些比官方示例更高级的示例。

Answer 2

由于libjpeg_turbo被编译为C库，而您的Android本机代码被编译为C++程序，所以您应该通过链接指令包含C库。

尝尝这个,

extern "C" {
#include "headers of libjpeg_turbo"
}

Answer 3

尝试修改您的Android.mk以包括您的libjpeg头部，也尝试找到一些有关dlopen()函数的信息来使用您编译的arm libjpeg.so。