轉載請標明出處：一片楓葉的專欄

首先說點題外話，對於想學android framework源代碼的同學，事實上能夠在github中fork一份，詳細地址：platform_frameworks_base
這裏面基本都是android framework層的源代碼了。並且近期發現了一個比較不錯的github插件：OctoTree，它 是一個瀏覽器插件，它能夠讓你在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，或者想看詳細的某個文件，這樣就會非常方便。

怎麽樣這樣查看源代碼的話是不是非常方面？

好了說一下我們今天須要介紹的Log對象，它位於android framework層utils包下，是一個final class類：查看其詳細定義：

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 [email protected] #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 [email protected] #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 [email protected] #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 [email protected] #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 [email protected] #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 [email protected] #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 [email protected] #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 [email protected] #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 [email protected] #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 [email protected] #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 [email protected] #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;
    }
}

能夠看到事實上final 類。所以我們不能通過繼承Log類的方式實現自身的日誌工具類，一般的我們能夠通過定義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誌級別，各自是：VERBOSE、DEBUG、INFO、WARN、ERROR、ASSERT等六種級別，可是我們平時使用的僅僅有前五種，即VERBOSE,DEBUG,INFO,WARN,ERROR。

通過查看源代碼我們發現Log類中全部的靜態日誌方法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的源代碼中查看，事實上其內部調用的是println_native方法。也就是通過JNI調用底層的c++輸出日誌；

我們臨時僅僅是分析到這裏。至於底層的c++日誌輸出的詳細實現不作分析。總結一下：

• Log.java是一個final類。所以我們不能夠繼承Log類來實現自己的日誌框架。可是能夠通過關聯（保存Log成員變量）的方式實現自己的Log工具類；

• Log.java中定義了六種日誌級別。可是我們通常僅僅是使用當中的五種日誌級別。分別相應著VERBOSE、DEBUG、INFO、WARN、ERROR，在詳細的使用場景下詳細分析；

• 有些同學對android自帶的日誌框架不太愜意。主要是無法定位日誌位置，這裏能夠查看我寫的一篇實現自己定義日誌框架的文章：github項目解析（五）–>android日誌框架

日誌能夠個性化的展示相關信息：

另外對android源代碼解析方法感興趣的可參考我的：
android源代碼解析之（一）–>android項目構建過程
android源代碼解析之（二）–>異步消息機制
android源代碼解析之（三）–>異步任務AsyncTask
android源代碼解析之（四）–>HandlerThread
android源代碼解析之（五）–>IntentService

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

Android源代碼解析之（六）-->Log日誌