android源碼解析之（五）--)Log相關介紹

這裡面基本都是android framework層的源碼了。而且最近發現了一個比較不錯的github插件：OctoTree，它 是一個浏覽器插件，它可以讓你在Github 看代碼時，左邊欄會出現一個樹狀結構，就像我們在IDE 一樣。當我們看一個項目的結構，或者想看具體的某個文件，這樣就會很方便。

怎麼樣這樣查看源代碼的話是不是很方面？<喎?/kf/ware/vc/" target="_blank" class="keylink">vcD4NCjxwPrrDwcvLtdK7z8LO0sPHvfHM7NDo0qq96cnctcRMb2e21M/zo6zL/M6709phbmRyb2lkIGZyYW1ld29ya7LjdXRpbHOw/M/Co6zKx9K7uPZmaW5hbCBjbGFzc8Dgo7qy6b+0xuS+38zltqjS5aO6PC9wPg0KPHByZSBjbGFzcz0="brush:java;"> 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; private Log() { } /** * Send a {@link #VERBOSE} 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. */ public static int v(String tag, String msg) { return println(LOG_ID_MAIN, VERBOSE, tag, msg); } /** * Send a {@link #VERBOSE} log message and log the exception. * @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. * @param tr An exception to log */ public static int v(String tag, String msg, Throwable tr) { return println(LOG_ID_MAIN, VERBOSE, tag, msg + '\n' + getStackTraceString(tr)); } /** * Send a {@link #DEBUG} 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. */ public static int d(String tag, String msg) { return println(LOG_ID_MAIN, DEBUG, tag, msg); } /** * Send a {@link #DEBUG} log message and log the exception. * @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. * @param tr An exception to log */ public static int d(String tag, String msg, Throwable tr) { return println(LOG_ID_MAIN, DEBUG, tag, msg + '\n' + getStackTraceString(tr)); } /** * Send an {@link #INFO} 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. */ public static int i(String tag, String msg) { return println(LOG_ID_MAIN, INFO, tag, msg); } /** * Send a {@link #INFO} log message and log the exception. * @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. * @param tr An exception to log */ public static int i(String tag, String msg, Throwable tr) { return println(LOG_ID_MAIN, INFO, tag, msg + '\n' + getStackTraceString(tr)); } /** * Send a {@link #WARN} 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. */ public static int w(String tag, String msg) { return println(LOG_ID_MAIN, WARN, tag, msg); } /** * Send a {@link #WARN} log message and log the exception. * @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. * @param tr An exception to log */ public static int w(String tag, String msg, Throwable tr) { return println(LOG_ID_MAIN, WARN, tag, msg + '\n' + getStackTraceString(tr)); } /* * Send a {@link #WARN} log message and log the exception. * @param tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. * @param tr An exception to log */ public static int w(String tag, Throwable tr) { return println(LOG_ID_MAIN, WARN, tag, getStackTraceString(tr)); } /** * Send an {@link #ERROR} 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. */ public static int e(String tag, String msg) { return println(LOG_ID_MAIN, ERROR, tag, msg); } /** * Send a {@link #ERROR} log message and log the exception. * @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. * @param tr An exception to log */ public static int e(String tag, String msg, Throwable tr) { return println(LOG_ID_MAIN, ERROR, tag, msg + '\n' + getStackTraceString(tr)); } /** * Handy function to get a loggable stack trace from a Throwable * @param tr An exception to log */ public static String getStackTraceString(Throwable tr) { if (tr == null) { return ""; } // This is to reduce the amount of log spew that apps do in the non-error // condition of the network being unavailable. Throwable t = tr; while (t != null) { if (t instanceof UnknownHostException) { return ""; } t = t.getCause(); } StringWriter sw = new StringWriter(); PrintWriter pw = new PrintWriter(sw); tr.printStackTrace(pw); pw.flush(); return sw.toString(); } /** * 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(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 final int LOG_ID_CRASH = 4; /** @hide */ @SuppressWarnings("unused") public static int println(int bufID, int priority, String tag, String msg) { return 0; } }

可以看到其實final 類，所以我們不能通過繼承Log類的方式實現自身的日志工具類，一般的我們可以通過定義Log成員變量的方式，封裝Log工具方法；

在Log類中我們定義了六種日志級別，分別是：VERBOSE、DEBUG、INFO、WARN、ERROR、ASSERT等六種級別，但是我們平時使用的只有前五種，即VERBOSE,DEBUG,INFO,WARN,ERROR。

通過查看源代碼我們發現Log類中所有的靜態日志方法Log.v()，Log.d()，Log.i()，Log.w()，Log.e()等方法都是底層都是調用了println方法，然後在github的源碼中查看，其實其內部調用的是println_native方法，也就是通過JNI調用底層的c++輸出日志；

我們暫時只是分析到這裡，至於底層的c++日志輸出的具體實現不作分析，總結一下：

Log.java是一個final類，所以我們不可以繼承Log類來實現自己的日志框架，但是可以通過關聯（保存Log成員變量）的方式實現自己的Log工具類；

Log.java中定義了六種日志級別，但是我們通常只是使用其中的五種日志級別，分別對應著VERBOSE、DEBUG、INFO、WARN、ERROR，在具體的使用場景下具體分析；

有些同學對android自帶的日志框架不太滿意，主要是無法定位日志位置。日志可以個性化的展示相關信息：