java - Android 如何通过 JNi 示例使用 libjpeg-turbo 库

Question

由于这篇文章，我成功地使用 ndk-build 构建了 libjpeg-turbo: libjpeg-turbo for android

我想在 libjpeg-turbo 的 example.c 中获得一个像 read_JPEG_file 这样的原生函数，以便从 Java 代码调用以将其用于 Android 应用。

有人能给我一个例子吗？如何使用 libjpeg-turbo ndk 内置库为 Java 编写本地方法？

我可以通过加载库

System.loadLibrary("libjpeg");

但接下来呢？该库没有任何可从 Java 调用的 native 方法。

我试图根据 JNI 文档编写一个 JNI c 类，但没有成功。示例代码非常适合学习如何操作。

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);
}

然后我用javah生成一个本地方法的C头文件，结果是com_test_app_NativeMethods.h文件包含:

/* 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，我在其中放置了 native 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;
}

文件 JPEGProcessing.c 和 com_test_app_NativeMethods.h 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，并在其中放置了这些行:

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)

从我的项目目录运行 ndk-build 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)
...

我不明白。如何将代码与 libjpeg-turbo 函数结合起来以获得工作库并在 java 中使用它？

我阅读了 NDK 说明和示例，但仍然不明白。

编辑:

带有简单 native 方法的 NDK 和 JNI 工作得很好。作为测试，我使用了以下效果很好的简单方法:

#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;
}

我用 libjpeg-turbo 试了一下，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

那么如何将原生代码与 ndk 构建的 libjpeg-turbo 库一起使用呢？

Answer 1

由于 libjpeg_turbo 被编译为 C 库，而您的 Android native 代码被编译为 C++ 程序，因此您应该通过链接指令包含 C 库。

试试这个，

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