Android源代码解析之（六）-->Log日志

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首先说点题外话，对于想学android framework源代码的同学，事实上能够在github中fork一份，详细地址：platform_frameworks_base
这里面基本都是android framework层的源代码了。并且近期发现了一个比較不错的github插件：OctoTree，它 是一个浏览器插件，它能够让你在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，或者想看详细的某个文件，这样就会非常方便。

怎么样这样查看源代码的话是不是非常方面？

好了说一下我们今天须要介绍的Log对象，它位于android framework层utils包下，是一个final class类：查看其详细定义：

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;
    }
}

能够看到事实上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，一般的我们能够通过定义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，各自是：VERBOSE、DEBUG、INFO、WARN、ERROR、ASSERT等六种级别，可是我们平时使用的仅仅有前五种，即VERBOSE,DEBUG,INFO,WARN,ERROR。

通过查看源代码我们发现Log类中全部的静态日志方法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的源代码中查看，事实上其内部调用的是println_native方法。也就是通过JNI调用底层的c++输出日志；

我们临时仅仅是分析到这里。至于底层的c++日志输出的详细实现不作分析。总结一下：

• Log.java是一个final类。所以我们不能够继承Log类来实现自己的日志框架。可是能够通过关联（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。可是我们通常仅仅是使用当中的五种日志级别。分别相应着VERBOSE、DEBUG、INFO、WARN、ERROR，在详细的使用场景下详细分析；

• 有些同学对android自带的日志框架不太惬意。主要是无法定位日志位置，这里能够查看我写的一篇实现自己定义日志框架的文章：github项目解析（五）–>android日志框架

日志能够个性化的展示相关信息：

另外对android源代码解析方法感兴趣的可參考我的：
android源代码解析之（一）–>android项目构建过程
android源代码解析之（二）–>异步消息机制
android源代码解析之（三）–>异步任务AsyncTask
android源代码解析之（四）–>HandlerThread
android源代码解析之（五）–>IntentService

本文以同步至github中：https://github.com/yipianfengye/androidSource，欢迎star和follow