Android应用程序框架层和系统运行库层日志系统源代码分析

最后更新于:2022-04-02 05:01:27

原文出处——>[Android应用程序框架层和系统运行库层日志系统源代码分析](http://blog.csdn.net/luoshengyang/article/details/6598703) 在开发Android应用程序时,少不了使用Log来监控和调试程序的执行。在上一篇文章Android日志系统驱动程序Logger源代码分析中,我们分析了驱动程序Logger的源代码,在前面的文章浅谈Android系统开发中Log的使用一文,我们也简单介绍在应用程序中使Log的方法,在这篇文章中,我们将详细介绍Android应用程序框架层和系统运行库存层日志系统的源代码,使得我们可以更好地理解Android的日志系统的实现。 我们在Android应用程序,一般是调用应用程序框架层的Java接口(android.util.Log)来使用日志系统,这个Java接口通过JNI方法和系统运行库最终调用内核驱动程序Logger把Log写到内核空间中。按照这个调用过程,我们一步步介绍Android应用程序框架层日志系统的源代码。学习完这个过程之后,我们可以很好地理解Android系统的架构,即应用程序层(Application)的接口是如何一步一步地调用到内核空间的。 **一. 应用程序框架层日志系统Java接口的实现。** 在浅谈Android系统开发中Log的使用一文中,我们曾经介绍过Android应用程序框架层日志系统的源代码接口。这里,为了描述方便和文章的完整性,我们重新贴一下这部份的代码,在**frameworks/base/core/java/android/util/Log.java**文件中,实现日志系统的Java接口: ~~~ ................................................ public final class Log { ................................................ /** * Priority constant for the println method; use Log.v. */ public static final int VERBOSE = 2; /** * Priority constant for the println method; use Log.d. */ public static final int DEBUG = 3; /** * Priority constant for the println method; use Log.i. */ public static final int INFO = 4; /** * Priority constant for the println method; use Log.w. */ public static final int WARN = 5; /** * Priority constant for the println method; use Log.e. */ public static final int ERROR = 6; /** * Priority constant for the println method. */ public static final int ASSERT = 7; ..................................................... public static int v(String tag, String msg) { return println_native(LOG_ID_MAIN, VERBOSE, tag, msg); } public static int v(String tag, String msg, Throwable tr) { return println_native(LOG_ID_MAIN, VERBOSE, tag, msg + '\n' + getStackTraceString(tr)); } public static int d(String tag, String msg) { return println_native(LOG_ID_MAIN, DEBUG, tag, msg); } public static int d(String tag, String msg, Throwable tr) { return println_native(LOG_ID_MAIN, DEBUG, tag, msg + '\n' + getStackTraceString(tr)); } public static int i(String tag, String msg) { return println_native(LOG_ID_MAIN, INFO, tag, msg); } public static int i(String tag, String msg, Throwable tr) { return println_native(LOG_ID_MAIN, INFO, tag, msg + '\n' + getStackTraceString(tr)); } public static int w(String tag, String msg) { return println_native(LOG_ID_MAIN, WARN, tag, msg); } public static int w(String tag, String msg, Throwable tr) { return println_native(LOG_ID_MAIN, WARN, tag, msg + '\n' + getStackTraceString(tr)); } public static int w(String tag, Throwable tr) { return println_native(LOG_ID_MAIN, WARN, tag, getStackTraceString(tr)); } public static int e(String tag, String msg) { return println_native(LOG_ID_MAIN, ERROR, tag, msg); } public static int e(String tag, String msg, Throwable tr) { return println_native(LOG_ID_MAIN, ERROR, tag, msg + '\n' + getStackTraceString(tr)); } .................................................................. /** @hide */ public static native int LOG_ID_MAIN = 0; /** @hide */ public static native int LOG_ID_RADIO = 1; /** @hide */ public static native int LOG_ID_EVENTS = 2; /** @hide */ public static native int LOG_ID_SYSTEM = 3; /** @hide */ public static native int println_native(int bufID, int priority, String tag, String msg); } ~~~ 定义了2~7一共6个日志优先级别ID和4个日志缓冲区ID。回忆一下Android日志系统驱动程序Logger源代码分析一文,在Logger驱动程序模块中,定义了log_main、log_events和log_radio三个日志缓冲区,分别对应三个设备文件/dev/log/main、/dev/log/events和/dev/log/radio。这里的4个日志缓冲区的前面3个ID就是对应这三个设备文件的文件描述符了,在下面的章节中,我们将看到这三个文件描述符是如何创建的。在下载下来的Android内核源代码中,第4个日志缓冲区LOG_ID_SYSTEM并没有对应的设备文件,在这种情况下,它和LOG_ID_MAIN对应同一个缓冲区ID,在下面的章节中,我们同样可以看到这两个ID是如何对应到同一个设备文件的。 在整个Log接口中,最关键的地方声明了println_native本地方法,所有的Log接口都是通过调用这个本地方法来实现Log的定入。下面我们就继续分析这个本地方法println_native。 **二. 应用程序框架层日志系统JNI方法的实现。** 在**frameworks/base/core/jni/android_util_Log.cpp**文件中,实现JNI方法println_native: ~~~ /* //device/libs/android_runtime/android_util_Log.cpp ** ** Copyright 2006, The Android Open Source Project ** ** Licensed under the Apache License, Version 2.0 (the "License"); ** you may not use this file except in compliance with the License. ** You may obtain a copy of the License at ** ** http://www.apache.org/licenses/LICENSE-2.0 ** ** Unless required by applicable law or agreed to in writing, software ** distributed under the License is distributed on an "AS IS" BASIS, ** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. ** See the License for the specific language governing permissions and ** limitations under the License. */ #define LOG_NAMESPACE "log.tag." #define LOG_TAG "Log_println" #include #include #include #include #include "jni.h" #include "utils/misc.h" #include "android_runtime/AndroidRuntime.h" #define MIN(a,b) ((aGetStringUTFChars(tag, NULL); if ((strlen(chars)+sizeof(LOG_NAMESPACE)) > PROPERTY_KEY_MAX) { jclass clazz = env->FindClass("java/lang/IllegalArgumentException"); char buf2[200]; snprintf(buf2, sizeof(buf2), "Log tag \"%s\" exceeds limit of %d characters\n", chars, PROPERTY_KEY_MAX - sizeof(LOG_NAMESPACE)); // release the chars! env->ReleaseStringUTFChars(tag, chars); env->ThrowNew(clazz, buf2); return false; } else { strncpy(key, LOG_NAMESPACE, sizeof(LOG_NAMESPACE)-1); strcpy(key + sizeof(LOG_NAMESPACE) - 1, chars); } env->ReleaseStringUTFChars(tag, chars); len = property_get(key, buf, ""); int logLevel = toLevel(buf); return (logLevel >= 0 && level >= logLevel) ? true : false; #endif /* HAVE_ANDROID_OS */ } /* * 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) { jclass npeClazz; npeClazz = env->FindClass("java/lang/NullPointerException"); assert(npeClazz != NULL); env->ThrowNew(npeClazz, "println needs a message"); return -1; } if (bufID < 0 || bufID >= LOG_ID_MAX) { jclass npeClazz; npeClazz = env->FindClass("java/lang/NullPointerException"); assert(npeClazz != NULL); env->ThrowNew(npeClazz, "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; } /* * JNI registration. */ static JNINativeMethod gMethods[] = { /* name, signature, funcPtr */ { "isLoggable", "(Ljava/lang/String;I)Z", (void*) android_util_Log_isLoggable }, { "println_native", "(IILjava/lang/String;Ljava/lang/String;)I", (void*) android_util_Log_println_native }, }; int register_android_util_Log(JNIEnv* env) { jclass clazz = env->FindClass("android/util/Log"); if (clazz == NULL) { LOGE("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)); } }; // namespace android ~~~ 在gMethods变量中,定义了println_native本地方法对应的函数调用是android_util_Log_println_native。在android_util_Log_println_native函数中,通过了各项参数验证正确后,就调用运行时库函数__android_log_buf_write来实现Log的写入操作。__android_log_buf_write函实实现在liblog库中,它有4个参数,分别缓冲区ID、优先级别ID、Tag字符串和Msg字符串。下面运行时库liblog中的__android_log_buf_write的实现。 **三. 系统运行库层日志系统的实现。** 在系统运行库层liblog库的实现中,内容比较多,这里,我们只关注日志写入操作__android_log_buf_write的相关实现: ~~~ int __android_log_buf_write(int bufID, int prio, const char *tag, const char *msg) { struct iovec vec[3]; if (!tag) tag = ""; /* XXX: This needs to go! */ if (!strcmp(tag, "HTC_RIL") || !strncmp(tag, "RIL", 3) || /* Any log tag with "RIL" as the prefix */ !strcmp(tag, "AT") || !strcmp(tag, "GSM") || !strcmp(tag, "STK") || !strcmp(tag, "CDMA") || !strcmp(tag, "PHONE") || !strcmp(tag, "SMS")) bufID = LOG_ID_RADIO; vec[0].iov_base = (unsigned char *) &prio; vec[0].iov_len = 1; vec[1].iov_base = (void *) tag; vec[1].iov_len = strlen(tag) + 1; vec[2].iov_base = (void *) msg; vec[2].iov_len = strlen(msg) + 1; return write_to_log(bufID, vec, 3); } ~~~ 函数首先是检查传进来的tag参数是否是为HTC_RIL、RIL、AT、GSM、STK、CDMA、PHONE和SMS中的一个,如果是,就无条件地使用ID为LOG_ID_RADIO的日志缓冲区作为写入缓冲区,接着,把传进来的参数prio、tag和msg分别存放在一个向量数组中,调用write_to_log函数来进入下一步操作。write_to_log是一个函数指针,定义在文件开始的位置上: ~~~ static int __write_to_log_init(log_id_t, struct iovec *vec, size_t nr); static int (*write_to_log)(log_id_t, struct iovec *vec, size_t nr) = __write_to_log_init; ~~~ 并且初始化为__write_to_log_init函数: ~~~ static int __write_to_log_init(log_id_t log_id, struct iovec *vec, size_t nr) { #ifdef HAVE_PTHREADS pthread_mutex_lock(&log_init_lock); #endif if (write_to_log == __write_to_log_init) { log_fds[LOG_ID_MAIN] = log_open("/dev/"LOGGER_LOG_MAIN, O_WRONLY); log_fds[LOG_ID_RADIO] = log_open("/dev/"LOGGER_LOG_RADIO, O_WRONLY); log_fds[LOG_ID_EVENTS] = log_open("/dev/"LOGGER_LOG_EVENTS, O_WRONLY); log_fds[LOG_ID_SYSTEM] = log_open("/dev/"LOGGER_LOG_SYSTEM, O_WRONLY); write_to_log = __write_to_log_kernel; if (log_fds[LOG_ID_MAIN] < 0 || log_fds[LOG_ID_RADIO] < 0 || log_fds[LOG_ID_EVENTS] < 0) { log_close(log_fds[LOG_ID_MAIN]); log_close(log_fds[LOG_ID_RADIO]); log_close(log_fds[LOG_ID_EVENTS]); log_fds[LOG_ID_MAIN] = -1; log_fds[LOG_ID_RADIO] = -1; log_fds[LOG_ID_EVENTS] = -1; write_to_log = __write_to_log_null; } if (log_fds[LOG_ID_SYSTEM] < 0) { log_fds[LOG_ID_SYSTEM] = log_fds[LOG_ID_MAIN]; } } #ifdef HAVE_PTHREADS pthread_mutex_unlock(&log_init_lock); #endif return write_to_log(log_id, vec, nr); } ~~~ 这里我们可以看到,如果是第一次调write_to_log函数,write_to_log == __write_to_log_init判断语句就会true,于是执行log_open函数打开设备文件,并把文件描述符保存在log_fds数组中。如果打开/dev/LOGGER_LOG_SYSTEM文件失败,即log_fds[LOG_ID_SYSTEM] < 0,就把log_fds[LOG_ID_SYSTEM]设置为log_fds[LOG_ID_MAIN],这就是我们上面描述的如果不存在ID为LOG_ID_SYSTEM的日志缓冲区,就把LOG_ID_SYSTEM设置为和LOG_ID_MAIN对应的日志缓冲区了。LOGGER_LOG_MAIN、LOGGER_LOG_RADIO、LOGGER_LOG_EVENTS和LOGGER_LOG_SYSTEM四个宏定义在system/core/include/cutils/logger.h文件中: ~~~ #define LOGGER_LOG_MAIN "log/main" #define LOGGER_LOG_RADIO "log/radio" #define LOGGER_LOG_EVENTS "log/events" #define LOGGER_LOG_SYSTEM "log/system" ~~~ 接着,把write_to_log函数指针指向__write_to_log_kernel函数: ~~~ static int __write_to_log_kernel(log_id_t log_id, struct iovec *vec, size_t nr) { ssize_t ret; int log_fd; if (/*(int)log_id >= 0 &&*/ (int)log_id < (int)LOG_ID_MAX) { log_fd = log_fds[(int)log_id]; } else { return EBADF; } do { ret = log_writev(log_fd, vec, nr); } while (ret < 0 && errno == EINTR); return ret; } ~~~ 函数调用log_writev来实现Log的写入,注意,这里通过一个循环来写入Log,直到写入成功为止。这里log_writev是一个宏,在文件开始的地方定义为: ~~~ #if FAKE_LOG_DEVICE // This will be defined when building for the host. #define log_open(pathname, flags) fakeLogOpen(pathname, flags) #define log_writev(filedes, vector, count) fakeLogWritev(filedes, vector, count) #define log_close(filedes) fakeLogClose(filedes) #else #define log_open(pathname, flags) open(pathname, flags) #define log_writev(filedes, vector, count) writev(filedes, vector, count) #define log_close(filedes) close(filedes) #endif ~~~ 这里,我们看到,一般情况下,log_writev就是writev了,这是个常见的批量文件写入函数,就不多说了。 至些,整个调用过程就结束了。总结一下,首先是从应用程序层调用应用程序框架层的Java接口,应用程序框架层的Java接口通过调用本层的JNI方法进入到系统运行库层的C接口,系统运行库层的C接口通过设备文件来访问内核空间层的Logger驱动程序。这是一个典型的调用过程,很好地诠释Android的系统架构,希望读者好好领会。
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