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探討:如何在NDK中呼叫Java的class

更新時間：2013年05月31日 10:18:00 作者：

本篇文章是對如何在NDK中呼叫Java的class進行了詳細的分析介紹，需要的朋友參考下

廢話不多說,直接上碼.......

package com.clouddevelop.cloudbox;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Paint.FontMetrics;
public class TextManager
{
    public Bitmap create(String text, float size)
    {
        try
        {
            Paint paint = new Paint();
            paint.setColor(Color.WHITE);
            paint.setTextSize(size);
            paint.setAlpha(255);
            paint.setFlags(Paint.ANTI_ALIAS_FLAG);
            paint.setTextAlign(Paint.Align.LEFT);
            paint.setAntiAlias(true);
            float[] widths = new float[text.length()];;
            paint.getTextWidths(text, widths);
            float width = 0;
            for(int i = 0 ;i < widths.length ; i++)
                width += widths[i];
            FontMetrics fm = paint.getFontMetrics();
            int mFontHeight = (int) (Math.ceil(fm.descent - fm.top) + 2);
            Bitmap textImg = Bitmap.createBitmap((int)width, mFontHeight, Bitmap.Config.ARGB_8888);
            Canvas c = new Canvas(textImg);
            if(fm.ascent < 0)
                c.drawText(text,0,(float)Math.abs(fm.ascent),paint);
            else
                c.drawText(text,0,fm.ascent* -1,paint);

            return textImg;
        }
        catch (Exception e) { }
        return null;
    }
    public int getWidth(Bitmap bmp) { return bmp.getWidth(); }
    public int getHeight(Bitmap bmp) { return bmp.getHeight(); }
    public void getPixels(Bitmap bmp, int[] pixels)
    {
        int w = bmp.getWidth();
        int h = bmp.getHeight();
        bmp.getPixels(pixels, 0, w, 0, 0, w, h);
    }
    public void close(Bitmap bmp)
    {
        bmp.recycle();
    }
}

要在NDK中呼叫Java的類,第一步當然要有一個Java的類,這個類是我自行建立
要產(chǎn)生一個文字的Bitmap,技術(shù)上沒什么復雜性,建立Paint,建立Bitmap然后用Canvas將文字寫入
在Canvas的drawText中,會使用FontMetrics的值來寫入文字,所以利用fm.ascent讓文字往上對齊

復制代碼代碼如下:

// declare
JNIEXPORT void JNICALL Java_com_clouddevelop_cloudbox_CloudRenderer_nativeTextInit
(JNIEnv* env, jclass cls, jobject textManager);
// implement
JNIEXPORT void JNICALL Java_com_clouddevelop_cloudbox_CloudRenderer_nativeTextInit
(JNIEnv* env, jclass cls, jobject textManager)
{
    g_env = env;
    g_textmgr = textManager;
    jclass business_class = env->GetObjectClass(g_textmgr);
    AndroidLog("initial textmanager success!");
}

接下來要在JNI中將JNIEnv存到全域變量中g(shù)_env

復制代碼代碼如下:

jobject getInstance(JNIEnv* env, jclass obj_class)
{
    jmethodID construction_id = env->GetMethodID(obj_class, "<init>", "()V");
    jobject obj = env->NewObject(obj_class, construction_id);
    return obj;
}
GLuint createText(const char* text, float size,float* rWidth, float* rHeight)
{
    if(g_env)
        AndroidLog("g_env exist");
    if(g_textmgr)
        AndroidLog("g_textmgr exist");
    jclass order_class = g_env->FindClass("com/clouddevelop/cloudbox/TextManager");
    AndroidLog("FindClass succeed");
    g_textmgr = getInstance(g_env, order_class);
    jclass cls = g_env->GetObjectClass(g_textmgr);
    AndroidLog("get class succeed");
    jmethodID mid;
    mid = g_env->GetMethodID(cls, "create",
                             "(Ljava/lang/String;F)Landroid/graphics/Bitmap;");
    AndroidLog("get create succeed");
    jstring data = g_env->NewStringUTF(text);
    jobject textImage = g_env->CallObjectMethod(g_textmgr, mid, data,size);
    AndroidLog("call create succeed");
    g_env->DeleteLocalRef(data);
    g_env->NewGlobalRef(textImage);
    /* Get image dimensions */
    mid = g_env->GetMethodID(cls, "getWidth", "(Landroid/graphics/Bitmap;)I");
    int width = g_env->CallIntMethod(g_textmgr, mid, textImage);
    AndroidLog("call getWidth succeed");
    mid = g_env->GetMethodID(cls, "getHeight", "(Landroid/graphics/Bitmap;)I");
    int height = g_env->CallIntMethod(g_textmgr, mid, textImage);
    AndroidLog("call getHeight succeed");
    *rWidth = width;
    *rHeight = height;
    /* Get pixels */
    jintArray image_data = g_env->NewIntArray(width * height);
    g_env->NewGlobalRef(image_data);
    mid = g_env->GetMethodID(cls, "getPixels", "(Landroid/graphics/Bitmap;[I)V");
    g_env->CallVoidMethod(g_textmgr, mid, textImage, image_data);
    AndroidLog("call getPixels succeed");
    jint *pixels = g_env->GetIntArrayElements(image_data, 0);
    //Now generate the OpenGL texture object
    GLuint texture;
    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA,
          GL_UNSIGNED_BYTE, (GLvoid*) pixels);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    AndroidLog("generate texture succeed");
    g_env->ReleaseIntArrayElements(image_data, pixels, 0);
    g_env->DeleteGlobalRef(image_data);
    /* Free image */
    mid = g_env->GetMethodID(cls, "close", "(Landroid/graphics/Bitmap;)V");
    g_env->CallVoidMethod(g_textmgr, mid, textImage);
    AndroidLog("call close succeed");
    g_env->DeleteGlobalRef(textImage);
    return texture;
}

復制代碼代碼如下:

jobject getInstance(JNIEnv* env, jclass obj_class)
{
    jmethodID construction_id = env->GetMethodID(obj_class, "<init>", "()V");
    jobject obj = env->NewObject(obj_class, construction_id);
    return obj;
}

上面這段代碼，是在native code中創(chuàng)建一個Java的實體類
這段代碼是我的CloudBox中創(chuàng)建文字紋理的代碼
mid = g_env->GetMethodID(cls, "getWidth", "(Landroid/graphics/Bitmap;)I");
int width = g_env->CallIntMethod(g_textmgr, mid, textImage);
其中這兩行,GetMethodID先取得該類的方法
在GetMethodID中第一參數(shù)是Java 類對象。第二個參數(shù)是參數(shù)（或方法名），第三個參數(shù)是該參數(shù)（或方法）的簽名。
那要如何取得方法的簽名呢？
我們要利用Javap -s TextManager這個指令來做

首先到.class所在的文件夾下，在我的范例中是在D:\CloudAndroid\CloudBox\CloudBoxAndroidGameApplication\bin\com\clouddevelop\cloudbox
然后鍵入javap -s TextManager就可以得到了,Signature就是我們要的簽名。