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 < 0 || 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) < 0) {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 < count; i++) {if (array[i].mProc(env) < 0) {
#ifndef NDEBUGALOGD("----------!!! %s failed to load\n", array[i].mName);
#endifreturn -1;}}return 0;
}

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 ($(HAVE_SELINUX),true)
LOCAL_C_INCLUDES += external/libselinux/include
LOCAL_SHARED_LIBRARIES += libselinux
LOCAL_CFLAGS += -DHAVE_SELINUX
endif # HAVE_SELINUXifeq ($(USE_OPENGL_RENDERER),true)LOCAL_SHARED_LIBRARIES += libhwui
endifLOCAL_SHARED_LIBRARIES += \libdl
# we need to access the private Bionic header
# <bionic_tls.h> in com_google_android_gles_jni_GLImpl.cpp
LOCAL_CFLAGS += -I$(LOCAL_PATH)/../../../../bionic/libc/privateLOCAL_LDLIBS += -lpthread -ldlifeq ($(WITH_MALLOC_LEAK_CHECK),true)LOCAL_CFLAGS += -DMALLOC_LEAK_CHECK
endifLOCAL_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_CHECK
endifLOCAL_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() < 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 = new Timer();timer.schedule(new TimerTask() {@Overridepublic void run() {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、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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