Android源码解析之（六）-->Log日志

2019年4月6日 255次阅读 来源: Android源码解析

转载请标明出处：一片枫叶的专栏

首先说点题外话，对于想学android framework源码的同学，其实可以在github中fork一份，具体地址：platform_frameworks_base
这里面基本都是android framework层的源码了。而且最近发现了一个比较不错的github插件：OctoTree，它 是一个浏览器插件，它可以让你在Github 看代码时，左边栏会出现一个树状结构，就像我们在IDE 一样。当我们看一个项目的结构，或者想看具体的某个文件，这样就会很方便。《Android源码解析之（六）-->Log日志》” /></p><p>怎么样这样查看源代码的话是不是很方面？</p><p>好了说一下我们今天需要介绍的Log对象，它位于android framework层utils包下，是一个final class类：查看其具体定义：</p><pre><code><span>public</span> <span>final</span> <span><span>class</span> <span>Log</span> {</span> <span>/** * Priority constant for the println method; use Log.v. */</span> <span>public</span> <span>static</span> <span>final</span> <span>int</span> VERBOSE = <span>2</span>; <span>/** * Priority constant for the println method; use Log.d. */</span> <span>public</span> <span>static</span> <span>final</span> <span>int</span> DEBUG = <span>3</span>; <span>/** * Priority constant for the println method; use Log.i. */</span> <span>public</span> <span>static</span> <span>final</span> <span>int</span> INFO = <span>4</span>; <span>/** * Priority constant for the println method; use Log.w. */</span> <span>public</span> <span>static</span> <span>final</span> <span>int</span> WARN = <span>5</span>; <span>/** * Priority constant for the println method; use Log.e. */</span> <span>public</span> <span>static</span> <span>final</span> <span>int</span> ERROR = <span>6</span>; <span>/** * Priority constant for the println method. */</span> <span>public</span> <span>static</span> <span>final</span> <span>int</span> ASSERT = <span>7</span>; <span>private</span> <span>Log</span>() { } <span>/** * Send a {@link #VERBOSE} log message. *<span> @param</span> tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. *<span> @param</span> msg The message you would like logged. */</span> <span>public</span> <span>static</span> <span>int</span> <span>v</span>(String tag, String msg) { <span>return</span> println(LOG_ID_MAIN, VERBOSE, tag, msg); } <span>/** * Send a {@link #VERBOSE} log message and log the exception. *<span> @param</span> tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. *<span> @param</span> msg The message you would like logged. *<span> @param</span> tr An exception to log */</span> <span>public</span> <span>static</span> <span>int</span> <span>v</span>(String tag, String msg, Throwable tr) { <span>return</span> println(LOG_ID_MAIN, VERBOSE, tag, msg + <span>'\n'</span> + getStackTraceString(tr)); } <span>/** * Send a {@link #DEBUG} log message. *<span> @param</span> tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. *<span> @param</span> msg The message you would like logged. */</span> <span>public</span> <span>static</span> <span>int</span> <span>d</span>(String tag, String msg) { <span>return</span> println(LOG_ID_MAIN, DEBUG, tag, msg); } <span>/** * Send a {@link #DEBUG} log message and log the exception. *<span> @param</span> tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. *<span> @param</span> msg The message you would like logged. *<span> @param</span> tr An exception to log */</span> <span>public</span> <span>static</span> <span>int</span> <span>d</span>(String tag, String msg, Throwable tr) { <span>return</span> println(LOG_ID_MAIN, DEBUG, tag, msg + <span>'\n'</span> + getStackTraceString(tr)); } <span>/** * Send an {@link #INFO} log message. *<span> @param</span> tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. *<span> @param</span> msg The message you would like logged. */</span> <span>public</span> <span>static</span> <span>int</span> <span>i</span>(String tag, String msg) { <span>return</span> println(LOG_ID_MAIN, INFO, tag, msg); } <span>/** * Send a {@link #INFO} log message and log the exception. *<span> @param</span> tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. *<span> @param</span> msg The message you would like logged. *<span> @param</span> tr An exception to log */</span> <span>public</span> <span>static</span> <span>int</span> <span>i</span>(String tag, String msg, Throwable tr) { <span>return</span> println(LOG_ID_MAIN, INFO, tag, msg + <span>'\n'</span> + getStackTraceString(tr)); } <span>/** * Send a {@link #WARN} log message. *<span> @param</span> tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. *<span> @param</span> msg The message you would like logged. */</span> <span>public</span> <span>static</span> <span>int</span> <span>w</span>(String tag, String msg) { <span>return</span> println(LOG_ID_MAIN, WARN, tag, msg); } <span>/** * Send a {@link #WARN} log message and log the exception. *<span> @param</span> tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. *<span> @param</span> msg The message you would like logged. *<span> @param</span> tr An exception to log */</span> <span>public</span> <span>static</span> <span>int</span> <span>w</span>(String tag, String msg, Throwable tr) { <span>return</span> println(LOG_ID_MAIN, WARN, tag, msg + <span>'\n'</span> + getStackTraceString(tr)); } <span>/* * 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 */</span> <span>public</span> <span>static</span> <span>int</span> <span>w</span>(String tag, Throwable tr) { <span>return</span> println(LOG_ID_MAIN, WARN, tag, getStackTraceString(tr)); } <span>/** * Send an {@link #ERROR} log message. *<span> @param</span> tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. *<span> @param</span> msg The message you would like logged. */</span> <span>public</span> <span>static</span> <span>int</span> <span>e</span>(String tag, String msg) { <span>return</span> println(LOG_ID_MAIN, ERROR, tag, msg); } <span>/** * Send a {@link #ERROR} log message and log the exception. *<span> @param</span> tag Used to identify the source of a log message. It usually identifies * the class or activity where the log call occurs. *<span> @param</span> msg The message you would like logged. *<span> @param</span> tr An exception to log */</span> <span>public</span> <span>static</span> <span>int</span> <span>e</span>(String tag, String msg, Throwable tr) { <span>return</span> println(LOG_ID_MAIN, ERROR, tag, msg + <span>'\n'</span> + getStackTraceString(tr)); } <span>/** * Handy function to get a loggable stack trace from a Throwable *<span> @param</span> tr An exception to log */</span> <span>public</span> <span>static</span> String <span>getStackTraceString</span>(Throwable tr) { <span>if</span> (tr == <span>null</span>) { <span>return</span> <span>; } // 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; } }

可以看到其实final 类，所以我们不能通过继承Log类的方式实现自身的日志工具类，一般的我们可以通过定义Log成员变量的方式，封装Log工具方法；

在Log类中我们定义了六种日志级别，分别是：VERBOSE、DEBUG、INFO、WARN、ERROR、ASSERT等六种级别，但是我们平时使用的只有前五种，即VERBOSE,DEBUG,INFO,WARN,ERROR。

通过查看源代码我们发现Log类中所有的静态日志方法Log.v()，Log.d()，Log.i()，Log.w()，Log.e()等方法都是底层都是调用了println方法，然后在github的源码中查看，其实其内部调用的是println_native方法，也就是通过JNI调用底层的c++输出日志；

我们暂时只是分析到这里，至于底层的c++日志输出的具体实现不作分析，总结一下：

  • Log.java是一个final类，所以我们不可以继承Log类来实现自己的日志框架，但是可以通过关联（保存Log成员变量）的方式实现自己的Log工具类；

  • Log.java中定义了六种日志级别，但是我们通常只是使用其中的五种日志级别，分别对应着VERBOSE、DEBUG、INFO、WARN、ERROR，在具体的使用场景下具体分析；

  • 有些同学对android自带的日志框架不太满意，主要是无法定位日志位置，这里可以查看我写的一篇实现自定义日志框架的文章：github项目解析（五）–>android日志框架

日志可以个性化的展示相关信息：
《Android源码解析之（六）-->Log日志》” /></p><p>另外对android源码解析方法感兴趣的可参考我的： <br /> android源码解析之（一）–>android项目构建过程 <br /> android源码解析之（二）–>异步消息机制 <br /> android源码解析之（三）–>异步任务AsyncTask <br /> android源码解析之（四）–>HandlerThread <br /> android源码解析之（五）–>IntentService</p><p><strong>本文以同步至github中：https://github.com/yipianfengye/androidSource，欢迎star和follow</strong></p></div><div style=     原文作者：Android源码解析
    原文地址: https://blog.csdn.net/qq_23547831/article/details/50963006
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