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Android 系统日志(Log) JNI实现流程源码分析
2026-10-30 17:22:14
来源：互联网转载
1、JNI概述
Java Native Interface (JNI) 是一种编程框架，使得Java代码能够与用其他编程语言（如C和C++）编写的本地代码进行交互。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。下面是对JNI机制的详细概述，包括其基本原理、工作流程、常见用途和示例代码。
1.1. JNI的基本原理
JNI的核心原理是通过一个标准化的接口，使得Java虚拟机（JVM）和本地代码可以互相通信和操作。JNI提供了一套函数，这些函数可以在本地代码中使用，以便与JVM交互，例如创建和操作Java对象，调用Java方法，处理异常等。
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、声明本地方法：在Java类中声明native方法。
2、加载本地库：在Java类中加载包含本地方法实现的库。
3、生成头文件：使用javac和javah工具生成包含native方法声明的头文件。
4、实现本地方法：在C/C++中实现native方法。
5、编译本地库：编译C/C++代码生成共享库。
6、调用本地方法：在Java代码中调用native方法。
1.3. JNI的常见用途
JNI常用于以下几种场景：
1、性能优化：将性能关键的部分用C/C++实现，以提高执行效率。
2、访问底层系统资源：访问操作系统的底层功能或硬件资源。
3、重用现有库：调用已有的C/C++库或API。
4、实现平台特定功能：在跨平台应用中实现特定平台的功能。
2、JNI的优缺点
2.1、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。
2、硬件访问：能够直接访问Java API不提供的底层硬件功能。
3、代码复用：重用现有的C/C++库，无需重新实现复杂的逻辑。
4、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。
2.2、缺点
1、复杂性增加：引入了额外的复杂性，需要了解C/C++和JNI API。
2、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。
3、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。
4、调试困难：调试JNI代码比纯Java代码困难，需要使用特定的工具和方法。
3、JNI的使用场景
1、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。
2、硬件功能：访问摄像头、传感器等底层硬件功能。
3、现有库：使用已有的C/C++库，例如图像处理库、加密库等。
4、跨语言调用：在需要与其他编程语言互操作时，例如从Java调用C++代码。
4、Android日志系统概述
Android日志系统主要由Log类和__android_log_print等C/C++函数组成，提供了记录调试信息的功能。Java层的日志API（如android.util.Log）最终调用的是本地日志函数，这些函数将日志消息写入系统日志缓冲区。
要深入了解Android Log的JNI实现流程，我们需要从JNI机制、Android日志系统、JNI方法实现及其相互配合的细节等方面进行详细分析。
5、本地方法声明（关键字native）
frameworks\base\core\java\android\util\Log.java
/** * Checks to see whether or not a log for the specified tag is loggable at the specified level. * * The default level of any tag is set to INFO. This means that any level above and including * INFO will be logged. Before you make any calls to a logging method you should check to see * if your tag should be logged. You can change the default level by setting a system property: * 'setprop log.tag.<YOUR_LOG_TAG> <LEVEL>' * Where level is either VERBOSE, DEBUG, INFO, WARN, ERROR, ASSERT, or SUPPRESS. SUPPRESS will * turn off all logging for your tag. You can also create a local.prop file that with the * following in it: * 'log.tag.<YOUR_LOG_TAG>=<LEVEL>' * and place that in /data/local.prop. * * @param tag The tag to check. * @param level The level to check. * @return Whether or not that this is allowed to be logged. * @throws IllegalArgumentException is thrown if the tag.length() > 23. */ public static native boolean isLoggable(String tag, int level);
/** * Low-level logging call. * @param priority The priority/type of this log message * @param tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. * @param msg The message you would like logged. * @return The number of bytes written. */ public static int println(int priority, String tag, String msg) { return println_native(LOG_ID_MAIN, priority, tag, msg); } /** @hide */ public static final int LOG_ID_MAIN = 0; /** @hide */ public static final int LOG_ID_RADIO = 1; /** @hide */ public static final int LOG_ID_EVENTS = 2; /** @hide */ public static final int LOG_ID_SYSTEM = 3; /** @hide */ public static native int println_native(int bufID, int priority, String tag, String msg); 6、本地方法实现
frameworks\base\core\jni\android_util_Log.cpp
static jboolean android_util_Log_isLoggable(JNIEnv* env, jobject clazz, jstring tag, jint level){ if (tag == NULL) { return false; } const char* chars = env->GetStringUTFChars(tag, NULL); if (!chars) { return false; } jboolean result = false; if ((strlen(chars)+sizeof(LOG_NAMESPACE)) > PROPERTY_KEY_MAX) { char buf2[200]; snprintf(buf2, sizeof(buf2), "Log tag \"%s\" exceeds limit of %d characters\n", chars, PROPERTY_KEY_MAX - sizeof(LOG_NAMESPACE)); jniThrowException(env, "java/lang/IllegalArgumentException", buf2); } else { result = isLoggable(chars, level); } env->ReleaseStringUTFChars(tag, chars); return result;} /* * In class android.util.Log: * public static native int println_native(int buffer, int priority, String tag, String msg) */static jint android_util_Log_println_native(JNIEnv* env, jobject clazz, jint bufID, jint priority, jstring tagObj, jstring msgObj){ const char* tag = NULL; const char* msg = NULL; if (msgObj == NULL) { jniThrowNullPointerException(env, "println needs a message"); return -1; } if (bufID = LOG_ID_MAX) { jniThrowNullPointerException(env, "bad bufID"); return -1; } if (tagObj != NULL) tag = env->GetStringUTFChars(tagObj, NULL); msg = env->GetStringUTFChars(msgObj, NULL); int res = __android_log_buf_write(bufID, (android_LogPriority)priority, tag, msg); if (tag != NULL) env->ReleaseStringUTFChars(tagObj, tag); env->ReleaseStringUTFChars(msgObj, msg); return res;} 7、虚拟机里面动态注册
frameworks\base\core\jni\AndroidRuntime.cpp
/* * Register android native functions with the VM. *//*static*/ int AndroidRuntime::startReg(JNIEnv* env){ /* * This hook causes all future threads created in this process to be * attached to the JavaVM. (This needs to go away in favor of JNI * Attach calls.) */ androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc); ALOGV("--- registering native functions ---\n"); /* * Every "register" function calls one or more things that return * a local reference (e.g. FindClass). Because we haven't really * started the VM yet, they're all getting stored in the base frame * and never released. Use Push/Pop to manage the storage. */ env->PushLocalFrame(200); if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) PopLocalFrame(NULL); return -1; } env->PopLocalFrame(NULL); //createJavaThread("fubar", quickTest, (void*) "hello"); return 0;} static int register_jni_procs(const RegJNIRec array[], size_t count, JNIEnv* env){ for (size_t i = 0; i 注意传入进来的参数
static const RegJNIRec gRegJNI[] = { REG_JNI(register_android_debug_JNITest), REG_JNI(register_com_android_internal_os_RuntimeInit), REG_JNI(register_android_os_SystemClock), REG_JNI(register_android_util_EventLog), REG_JNI(register_android_util_Log), REG_JNI(register_android_util_FloatMath), REG_JNI(register_android_text_format_Time), REG_JNI(register_android_content_AssetManager), REG_JNI(register_android_content_StringBlock), REG_JNI(register_android_content_XmlBlock), REG_JNI(register_android_emoji_EmojiFactory), REG_JNI(register_android_text_AndroidCharacter), REG_JNI(register_android_text_AndroidBidi), REG_JNI(register_android_view_InputDevice), REG_JNI(register_android_view_KeyCharacterMap), REG_JNI(register_android_os_Process), REG_JNI(register_android_os_SystemProperties), REG_JNI(register_android_os_Binder), REG_JNI(register_android_os_Parcel), REG_JNI(register_android_view_DisplayEventReceiver), REG_JNI(register_android_nio_utils), REG_JNI(register_android_graphics_PixelFormat), REG_JNI(register_android_graphics_Graphics), REG_JNI(register_android_view_GLES20DisplayList), REG_JNI(register_android_view_GLES20Canvas), REG_JNI(register_android_view_HardwareRenderer), REG_JNI(register_android_view_Surface), REG_JNI(register_android_view_SurfaceSession), REG_JNI(register_android_view_TextureView), REG_JNI(register_com_google_android_gles_jni_EGLImpl), REG_JNI(register_com_google_android_gles_jni_GLImpl), REG_JNI(register_android_opengl_jni_EGL14), REG_JNI(register_android_opengl_jni_GLES10), REG_JNI(register_android_opengl_jni_GLES10Ext), REG_JNI(register_android_opengl_jni_GLES11), REG_JNI(register_android_opengl_jni_GLES11Ext), REG_JNI(register_android_opengl_jni_GLES20), REG_JNI(register_android_graphics_Bitmap), REG_JNI(register_android_graphics_BitmapFactory), REG_JNI(register_android_graphics_BitmapRegionDecoder), REG_JNI(register_android_graphics_Camera), REG_JNI(register_android_graphics_Canvas), REG_JNI(register_android_graphics_ColorFilter), REG_JNI(register_android_graphics_DrawFilter), REG_JNI(register_android_graphics_Interpolator), REG_JNI(register_android_graphics_LayerRasterizer), REG_JNI(register_android_graphics_MaskFilter), REG_JNI(register_android_graphics_Matrix), REG_JNI(register_android_graphics_Movie), REG_JNI(register_android_graphics_NinePatch), REG_JNI(register_android_graphics_Paint), REG_JNI(register_android_graphics_Path), REG_JNI(register_android_graphics_PathMeasure), REG_JNI(register_android_graphics_PathEffect), REG_JNI(register_android_graphics_Picture), REG_JNI(register_android_graphics_PorterDuff), REG_JNI(register_android_graphics_Rasterizer), REG_JNI(register_android_graphics_Region), REG_JNI(register_android_graphics_Shader), REG_JNI(register_android_graphics_SurfaceTexture), REG_JNI(register_android_graphics_Typeface), REG_JNI(register_android_graphics_Xfermode), REG_JNI(register_android_graphics_YuvImage), REG_JNI(register_android_database_CursorWindow), REG_JNI(register_android_database_SQLiteConnection), REG_JNI(register_android_database_SQLiteGlobal), REG_JNI(register_android_database_SQLiteDebug), REG_JNI(register_android_os_Debug), REG_JNI(register_android_os_FileObserver), REG_JNI(register_android_os_FileUtils), REG_JNI(register_android_os_MessageQueue), REG_JNI(register_android_os_ParcelFileDescriptor), REG_JNI(register_android_os_SELinux), REG_JNI(register_android_os_Trace), REG_JNI(register_android_os_UEventObserver), REG_JNI(register_android_net_LocalSocketImpl), REG_JNI(register_android_net_NetworkUtils), REG_JNI(register_android_net_TrafficStats), REG_JNI(register_android_net_wifi_WifiManager), REG_JNI(register_android_os_MemoryFile), REG_JNI(register_com_android_internal_os_ZygoteInit), REG_JNI(register_android_hardware_Camera), REG_JNI(register_android_hardware_SensorManager), REG_JNI(register_android_hardware_SerialPort), REG_JNI(register_android_hardware_UsbDevice), REG_JNI(register_android_hardware_UsbDeviceConnection), REG_JNI(register_android_hardware_UsbRequest), REG_JNI(register_android_media_AudioRecord), REG_JNI(register_android_media_AudioSystem), REG_JNI(register_android_media_AudioTrack), REG_JNI(register_android_media_JetPlayer), REG_JNI(register_android_media_RemoteDisplay), REG_JNI(register_android_media_ToneGenerator), REG_JNI(register_android_opengl_classes), REG_JNI(register_android_server_NetworkManagementSocketTagger), REG_JNI(register_android_server_Watchdog), REG_JNI(register_android_ddm_DdmHandleNativeHeap), REG_JNI(register_android_backup_BackupDataInput), REG_JNI(register_android_backup_BackupDataOutput), REG_JNI(register_android_backup_FileBackupHelperBase), REG_JNI(register_android_backup_BackupHelperDispatcher), REG_JNI(register_android_app_backup_FullBackup), REG_JNI(register_android_app_ActivityThread), REG_JNI(register_android_app_NativeActivity), REG_JNI(register_android_view_InputChannel), REG_JNI(register_android_view_InputEventReceiver), REG_JNI(register_android_view_KeyEvent), REG_JNI(register_android_view_MotionEvent), REG_JNI(register_android_view_PointerIcon), REG_JNI(register_android_view_VelocityTracker), REG_JNI(register_android_content_res_ObbScanner), REG_JNI(register_android_content_res_Configuration), REG_JNI(register_android_animation_PropertyValuesHolder), REG_JNI(register_com_android_internal_content_NativeLibraryHelper),}; 看到register_android_util_Log了吧
int register_android_util_Log(JNIEnv* env){ jclass clazz = env->FindClass("android/util/Log"); if (clazz == NULL) { ALOGE("Can't find android/util/Log"); return -1; } levels.verbose = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "VERBOSE", "I")); levels.debug = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "DEBUG", "I")); levels.info = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "INFO", "I")); levels.warn = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "WARN", "I")); levels.error = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "ERROR", "I")); levels.assert = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "ASSERT", "I")); return AndroidRuntime::registerNativeMethods(env, "android/util/Log", gMethods, NELEM(gMethods));} 动态注册完成，那共享库（.so）是在哪里加载的呢？
8、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# 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_***.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
public static void main(String[] args) { if (System.currentTimeMillis() 现在知道Log的JNI实现流程了吧？！！！
9、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。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。理解JNI的工作原理和使用方法，对于开发高性能、功能丰富的Android应用至关重要。
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。在Android开发中，JNI广泛应用于性能优化、硬件访问以及重用现有的本地库。以下是关于JNI使用的总结，包括其优缺点、使用场景、开发步骤、注意事项以及最佳实践。
Android日志系统从应用程序层到JNI层、本地层、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。通过分析各个层级的代码实现，可以更深入地理解日志系统的设计和工作原理。这些组件紧密协作，提供了一个高效、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。
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