Android Log 源码介绍

1.关于Log源码介绍

• Log对象，它位于android framework层utils包下，是一个final class类

  • 源码如下所示，可以看到其实final类，所以不能通过继承Log类的方式实现自身的日志工具类，一般可以通过定义Log成员变量的方式，封装Log工具方法。

  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;
      }
  }

2.Log日志的种类

• Android中日志级别有哪几种？
  • 1.Log.v 的输出颜色为黑色的，输出大于或等于VERBOSE日志级别的信息，也就是可见级别，一般是最低的信息提示
  • 2.Log.d的输出颜色是蓝色的，也就是调式级别，一般不会中止程序，一般是程序员为了调试而打印的log
  • 3.Log.i的输出为绿色，输出大于或等于INFO日志级别的信息，也就是信息界级别，不会中止程序，一般是系统中执行操作的信息提示
  • 4.Log.w的输出为橙色, 输出大于或等于WARN日志级别的信息，也就是警告级别，一般不会中止程序，但是可能会影响程序执行结果
  • 5.Log.e的输出为红色，仅输出ERROR日志级别的信息，也就是错误级别，一般会中止程序运行，是最严重的Log级别。
  • 解释：
    • verbose
    • debug调试
    • info信息
    • warn警告
    • error误差
• 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。