TwoLibs 示例
隨 Android NDK 提供的另外一個例子 TwoLibs,其中有兩個庫,一個為動態庫,一個為靜態庫,最終供 Android Application 使用的動態庫使用靜態庫中的函數,如下圖所示:
其中在 first.c 中定義了一個簡單的 C 函數
int first(int x, int y)
{
return x+y;
}
second.c 調用這個函數
jint
Java_com_example_twolibs_TwoLibs_add( JNIEnv* env,
jobject this,
jint x,
jint y )
{
return first(x, y);
}
為了介紹動態庫調用靜態庫的方法,這個例子將兩個 C 文件編譯成兩個模塊,這是通過 android.mk 來定義的
LOCAL_PATH:= $(call my-dir)
# first lib, which will be built statically
#
include $(CLEAR_VARS)
LOCAL_MODULE:= libtwolib-first
LOCAL_SRC_FILES := first.c
include $(BUILD_STATIC_LIBRARY)
# second lib, which will depend on and include the first one
#
include $(CLEAR_VARS)
LOCAL_MODULE:= libtwolib-second
LOCAL_SRC_FILES := second.c
LOCAL_STATIC_LIBRARIES := libtwolib-first
include $(BUILD_SHARED_LIBRARY)
註:當然對於這個簡單的例子,大可不必編譯成兩個模塊。
在 Android 應用中調用動態庫(Android 應用只可以調用動態庫)
public class TwoLibs extends Activity
{
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
TextView tv = new TextView(this);
int x = 1000;
int y = 42;
// here, we dynamically load the library at runtime
// before calling the native method.
//
System.loadLibrary("twolib-second");
int z = add(x, y);
tv.setText( "The sum of " + x + " and " + y + " is " + z );
setContentView(tv);
}
public native int add(int x, int y);
}
下面就可以使用 ndk-build ,編譯 C 代碼,然後再使用 Eclipse 編譯 Java 代碼,運行結果如下:
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