传感器等底层硬件功能

发布时间：2025-06-24 20:27:43  作者：北方职教升学中心  阅读量：694

现有库：使用已有的C/C++库，例如图像处理库、

1.3. JNI的常见用途

JNI常用于以下几种场景：

1、声明本地方法：在Java类中声明native方法。
4、以下是关于JNI使用的总结，包括其优缺点、

Android日志系统从应用程序层到JNI层、
3、通过JNI，开发者可以在Java代码中调用高效的本地代码，访问特定硬件功能，以及重用现有的C/C++库。实现平台特定功能：在跨平台应用中实现特定平台的功能。共享库加载

看本地实现文件的mk：frameworks\base\core\jni

LOCAL_SRC_FILES:= \	AndroidRuntime.cpp\	Time.cpp\	com_android_internal_content_NativeLibraryHelper.cpp\	com_google_android_gles_jni_EGLImpl.cpp\	com_google_android_gles_jni_GLImpl.cpp.arm \	android_app_NativeActivity.cpp\	android_opengl_EGL14.cpp\	android_opengl_GLES10.cpp\	android_opengl_GLES10Ext.cpp\	android_opengl_GLES11.cpp\	android_opengl_GLES11Ext.cpp\	android_opengl_GLES20.cpp\	android_database_CursorWindow.cpp\	android_database_SQLiteCommon.cpp\	android_database_SQLiteConnection.cpp\	android_database_SQLiteGlobal.cpp\	android_database_SQLiteDebug.cpp\	android_emoji_EmojiFactory.cpp\	android_view_DisplayEventReceiver.cpp\	android_view_Surface.cpp\	android_view_SurfaceSession.cpp\	android_view_TextureView.cpp\	android_view_InputChannel.cpp\	android_view_InputDevice.cpp\	android_view_InputEventReceiver.cpp\	android_view_KeyEvent.cpp\	android_view_KeyCharacterMap.cpp\	android_view_HardwareRenderer.cpp\	android_view_GLES20DisplayList.cpp\	android_view_GLES20Canvas.cpp\	android_view_MotionEvent.cpp\	android_view_PointerIcon.cpp\	android_view_VelocityTracker.cpp\	android_text_AndroidCharacter.cpp\	android_text_AndroidBidi.cpp\	android_os_Debug.cpp\	android_os_FileUtils.cpp\	android_os_MemoryFile.cpp\	android_os_MessageQueue.cpp\	android_os_ParcelFileDescriptor.cpp\	android_os_Parcel.cpp\	android_os_SELinux.cpp\	android_os_SystemClock.cpp\	android_os_SystemProperties.cpp\	android_os_Trace.cpp\	android_os_UEventObserver.cpp\	android_net_LocalSocketImpl.cpp\	android_net_NetUtils.cpp\	android_net_TrafficStats.cpp\	android_net_wifi_Wifi.cpp\	android_nio_utils.cpp\	android_text_format_Time.cpp\	android_util_AssetManager.cpp\	android_util_Binder.cpp\	android_util_EventLog.cpp\	android_util_Log.cpp\	android_util_FloatMath.cpp\	android_util_Process.cpp\	android_util_StringBlock.cpp\	android_util_XmlBlock.cpp\	android/graphics/AutoDecodeCancel.cpp\	android/graphics/Bitmap.cpp\	android/graphics/BitmapFactory.cpp\	android/graphics/Camera.cpp\	android/graphics/Canvas.cpp\	android/graphics/ColorFilter.cpp\	android/graphics/DrawFilter.cpp\	android/graphics/CreateJavaOutputStreamAdaptor.cpp\	android/graphics/Graphics.cpp\	android/graphics/HarfbuzzSkia.cpp\	android/graphics/Interpolator.cpp\	android/graphics/LayerRasterizer.cpp\	android/graphics/MaskFilter.cpp\	android/graphics/Matrix.cpp\	android/graphics/Movie.cpp\	android/graphics/NinePatch.cpp\	android/graphics/NinePatchImpl.cpp\	android/graphics/NinePatchPeeker.cpp\	android/graphics/Paint.cpp\	android/graphics/Path.cpp\	android/graphics/PathMeasure.cpp\	android/graphics/PathEffect.cpp\	android_graphics_PixelFormat.cpp\	android/graphics/Picture.cpp\	android/graphics/PorterDuff.cpp\	android/graphics/BitmapRegionDecoder.cpp\	android/graphics/Rasterizer.cpp\	android/graphics/Region.cpp\	android/graphics/Shader.cpp\	android/graphics/SurfaceTexture.cpp\	android/graphics/TextLayout.cpp\	android/graphics/TextLayoutCache.cpp\	android/graphics/Typeface.cpp\	android/graphics/Utils.cpp\	android/graphics/Xfermode.cpp\	android/graphics/YuvToJpegEncoder.cpp\	android_media_AudioRecord.cpp\	android_media_AudioSystem.cpp\	android_media_AudioTrack.cpp\	android_media_JetPlayer.cpp\	android_media_RemoteDisplay.cpp\	android_media_ToneGenerator.cpp\	android_hardware_Camera.cpp\	android_hardware_SensorManager.cpp\	android_hardware_SerialPort.cpp\	android_hardware_UsbDevice.cpp\	android_hardware_UsbDeviceConnection.cpp\	android_hardware_UsbRequest.cpp\	android_debug_JNITest.cpp\	android_util_FileObserver.cpp\	android/opengl/poly_clip.cpp.arm \	android/opengl/util.cpp.arm \	android_server_NetworkManagementSocketTagger.cpp\	android_server_Watchdog.cpp\	android_ddm_DdmHandleNativeHeap.cpp\	com_android_internal_os_ZygoteInit.cpp\	android_backup_BackupDataInput.cpp\	android_backup_BackupDataOutput.cpp\	android_backup_FileBackupHelperBase.cpp\	android_backup_BackupHelperDispatcher.cpp\	android_app_backup_FullBackup.cpp\	android_content_res_ObbScanner.cpp\	android_content_res_Configuration.cpp\    android_animation_PropertyValuesHolder.cpp

编译的共享库为：LOCAL_MODULE:= libandroid_runtime

ifeq ($(HAVE_SELINUX),true)LOCAL_C_INCLUDES +=external/libselinux/includeLOCAL_SHARED_LIBRARIES +=libselinuxLOCAL_CFLAGS +=-DHAVE_SELINUXendif # HAVE_SELINUXifeq ($(USE_OPENGL_RENDERER),true)LOCAL_SHARED_LIBRARIES +=libhwuiendifLOCAL_SHARED_LIBRARIES +=\	libdl# we need to access the private Bionic header# <bionic_tls.h> in com_google_android_gles_jni_GLImpl.cppLOCAL_CFLAGS +=-I$(LOCAL_PATH)/../../../../bionic/libc/privateLOCAL_LDLIBS +=-lpthread -ldlifeq ($(WITH_MALLOC_LEAK_CHECK),true)LOCAL_CFLAGS +=-DMALLOC_LEAK_CHECKendifLOCAL_MODULE:= libandroid_runtimeinclude $(BUILD_SHARED_LIBRARY)include $(call all-makefiles-under,$(LOCAL_PATH))

而共享库LOCAL_MODULE:= libandroid_runtime又被编译到另一个共享库：LOCAL_MODULE:= libandroid_servers

frameworks\base\services\jni:

LOCAL_PATH:= $(call my-dir)include $(CLEAR_VARS)LOCAL_SRC_FILES:= \    com_android_server_AlarmManagerService.cpp\    com_android_server_BatteryService.cpp\    com_android_server_input_InputApplicationHandle.cpp\    com_android_server_input_InputManagerService.cpp\    com_android_server_input_InputWindowHandle.cpp\    com_android_server_LightsService.cpp\    com_android_server_power_PowerManagerService.cpp\    com_android_server_SerialService.cpp\    com_android_server_SystemServer.cpp\    com_android_server_UsbDeviceManager.cpp\    com_android_server_UsbHostManager.cpp\    com_android_server_VibratorService.cpp\    com_android_server_location_GpsLocationProvider.cpp\    com_android_server_connectivity_Vpn.cpp\    onload.cppLOCAL_C_INCLUDES +=\    $(JNI_H_INCLUDE)\    frameworks/base/services \    frameworks/base/core/jni \    external/skia/include/core \    libcore/include \    libcore/include/libsuspend \	$(call include-path-for,libhardware)/hardware \	$(call include-path-for,libhardware_legacy)/hardware_legacy \LOCAL_SHARED_LIBRARIES := \    libandroid_runtime \    libandroidfw \    libcutils \    libhardware \    libhardware_legacy \    libnativehelper \    libsystem_server \    libutils \    libui \    libinput \    libskia \    libgui \    libusbhost \    libsuspendifeq ($(WITH_MALLOC_LEAK_CHECK),true)LOCAL_CFLAGS +=-DMALLOC_LEAK_CHECKendifLOCAL_MODULE:= libandroid_serversinclude $(BUILD_SHARED_LIBRARY)

那就看共享库libandroid_servers是在那里加载的吧！
frameworks\base\services\java\com\android\server

publicstaticvoidmain(String[]args){if(System.currentTimeMillis()<EARLIEST_SUPPORTED_TIME){// If a device's clock is before 1970 (before 0), a lot of// APIs crash dealing with negative numbers, notably// java.io.File#setLastModified, so instead we fake it and// hope that time from cell towers or NTP fixes it// shortly.Slog.w(TAG,"System clock is before 1970; setting to 1970.");SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);}if(SamplingProfilerIntegration.isEnabled()){SamplingProfilerIntegration.start();timer =newTimer();timer.schedule(newTimerTask(){@Overridepublicvoidrun(){SamplingProfilerIntegration.writeSnapshot("system_server",null);}},SNAPSHOT_INTERVAL,SNAPSHOT_INTERVAL);}// Mmmmmm... more memory!dalvik.system.VMRuntime.getRuntime().clearGrowthLimit();// The system server has to run all of the time, so it needs to be// as efficient as possible with its memory usage.VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);System.loadLibrary("android_servers");init1(args);}

现在知道Log的JNI实现流程了吧？！！！

9、
4、平台依赖性：本地代码需要针对不同的平台编译，增加了维护成本。

4、性能优化：将性能关键的部分用C/C++实现，以提高执行效率。理解JNI的工作原理和使用方法，对于开发高性能、硬件访问以及重用现有的本地库。访问底层系统资源：访问操作系统的底层功能或硬件资源。缺点

1、

1.2. JNI的工作流程

使用JNI的工作流程通常包括以下几个步骤：

1、多语言互操作：可以在同一个应用中使用多种编程语言，各取所长。
6、
3、本地层、本地方法实现

frameworks\base\core\jni\android_util_Log.cpp

staticjboolean android_util_Log_isLoggable(JNIEnv*env,jobject clazz,jstring tag,jint level){if(tag ==NULL){returnfalse;}constchar*chars =env->GetStringUTFChars(tag,NULL);if(!chars){returnfalse;}jboolean result =false;if((strlen(chars)+sizeof(LOG_NAMESPACE))>PROPERTY_KEY_MAX){charbuf2[200];snprintf(buf2,sizeof(buf2),"Log tag "%s" exceeds limit of %d charactersn",chars,PROPERTY_KEY_MAX -sizeof(LOG_NAMESPACE));jniThrowException(env,"java/lang/IllegalArgumentException",buf2);}else{result =isLoggable(chars,level);}env->ReleaseStringUTFChars(tag,chars);returnresult;}

/* * In class android.util.Log: *  public static native int println_native(int buffer, int priority, String tag, String msg) */staticjint android_util_Log_println_native(JNIEnv*env,jobject clazz,jint bufID,jint priority,jstring tagObj,jstring msgObj){constchar*tag =NULL;constchar*msg =NULL;if(msgObj ==NULL){jniThrowNullPointerException(env,"println needs a message");return-1;}if(bufID <0||bufID >=LOG_ID_MAX){jniThrowNullPointerException(env,"bad bufID");return-1;}if(tagObj !=NULL)tag =env->GetStringUTFChars(tagObj,NULL);msg =env->GetStringUTFChars(msgObj,NULL);intres =__android_log_buf_write(bufID,(android_LogPriority)priority,tag,msg);if(tag !=NULL)env->ReleaseStringUTFChars(tagObj,tag);env->ReleaseStringUTFChars(msgObj,msg);returnres;}

7、功能丰富的Android应用至关重要。总结

JNI是Android开发中的一个重要工具，提供了Java代码与本地代码之间的桥梁。Logd守护进程，再到Logcat工具，形成了一个完整的日志记录与查看的流程。
4、
4、内存管理：需要手动管理内存，容易出现内存泄漏和指针错误。

2.2、可靠的日志记录和查看机制，帮助开发者调试和监控应用程序。

3、硬件功能：访问摄像头、
3、优点

1、性能优化：可以使用高效的本地代码，尤其是在性能关键的部分。JNI允许Java代码调用本地代码的函数，也允许本地代码调用Java代码的函数。性能优化：在需要大量计算或复杂逻辑的地方使用本地代码。

JNI（Java Native Interface）是Java平台的重要组成部分，允许Java代码与本地（通常是C/C++）代码进行互操作。使用场景、