随Android NDK 提供的另外一个例子TwoLibs,其中有两个库,一个为动态库,一个为静态库,最终供Android Application使用的动态库使用静态库中的函数,如下图所示:
其中在first.c 中定义了一个简单的C函数
int first(int x, int y)
{
return x+y;
}
second.c 调用这个函数
jint
Java_com_example_twolibs_TwoLibs_add( JNIEnv* env,
jobject this,
jint x,
jint y )
{
return first(x, y);
}
为了介绍动态库调用静态库的方法,这个例子将两个C文件编译成两个模块,这是通过android.mk 来定义的。
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)
注:当然对于这个简单的例子,大可不必编译成两个模块。
在Android 应用中调用动态库(Android应用只可以调用动态库)
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 ,编译C代码,然后再使用Eclipse编译Java代码,运行结果如下: