JNI编程初试

public class TestMain
{
    public native boolean printInfo();

    public native int getNum();

    public native void setNum(int num);

 

三个方法，第一个方法用于打印“hello world!”到屏幕。第二个方法用于获取一个全局变量的值，第三个方法用于设置全局变量的值。

找到eclipse输出的class文件的所在的包的位置，我的目录是在“F:\java_code\TestJNI\bin”，然后执行如下命令：

F:\java_code\TestJNI\bin>javah com.jni.test.TestMain

这样会在bin目录下生成一个“com_jni_test_TestMain.h”文件。

新建VC++动态链接库工程，在工程的包含目录中新增jvm的头文件。如下图所示：

 
添加.cpp文件，编写三个函数的实现：

#include "com_jni_test_TestMain.h"

static int g_num = 0;

jboolean JNICALL Java_com_jni_test_TestMain_printInfo(JNIEnv *, jobject)
{
    printf("hello world!\n");
    return true;
}

jint JNICALL Java_com_jni_test_TestMain_getNum(JNIEnv *, jobject)
{

    return g_num;
}


void JNICALL Java_com_jni_test_TestMain_setNum(JNIEnv *, jobject, jint num)
{
    g_num = num;
}

使用VC环境编译出dll库文件。

使用如下三种方法加载动态库文件：

1）使用绝对路径

将TestJNI.dll库文件放置在F:\java_code\TestJNI\bin\目录下。获取该文件的绝对路径来加载：

private static void loadlibrary()

{

    System.load(System.getProperty("user.dir") + File.separator + "bin" + File.separator + "TestJNI.dll");

}

private static void loadlibrary2() throws UnsatisfiedLinkError
{
    try
    {
        String libpath = System.getProperty("java.library.path");
        if (libpath == null || libpath.length() == 0)
        {
            throw new RuntimeException("java.library.path is null");
        }
        String path = null;
        StringTokenizer st = new StringTokenizer(libpath, System.getProperty("path.separator"));
        if (st.hasMoreElements())
        {
            path = st.nextToken();
        }
        else
        {
            throw new RuntimeException("can not split library path:" + libpath);
        }
        Class<TestMain> thisClass = TestMain.class;
        InputStream inputStream = thisClass.getResource("TestJNI.dll").openStream();
        File dllFile = new File(new File(path), "TestMain.dll");
        if (!dllFile.exists())
        {
            FileOutputStream outputStream = new FileOutputStream(dllFile);
            byte[] array = new byte[8192];
            for (int i = inputStream.read(array); i != -1; i = inputStream.read(array))
            {
                outputStream.write(array, 0, i);
            }
            outputStream.close();
        }
        System.loadLibrary("TestMain");
    }
    catch (UnsatisfiedLinkError e)
    {
        throw e;
    }
    catch (Throwable e)
    {
        throw new RuntimeException("load RegistryUtil.dll error!", e);
    }
}

private static void loadlibrary3()
{
    URL classURL = TestMain.class.getResource("TestMain.class");
    String classPath = new File(classURL.getPath()).getParent();
    String syslibpath = System.getProperty("java.library.path") + classPath + ";";
    System.setProperty("java.library.path", syslibpath);
    for(String path: System.getProperty("java.library.path").split(";"))
    {
        System.out.println(path);
    }
    System.loadLibrary("TestJNI");
}

public static void main(String[] args)
{
    TestMain handle = new TestMain();
    handle.printInfo();
    handle.setNum(10000);
    System.out.println(handle.getNum());
}