Log.isLoggable的使用

Posted 2022-11-30 他叫小黑

本文将介绍怎么打印出framework源码中的log。

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在android源码中，经常可以看到如下的log代码：

//Audioservice.java

private static final String TAG = "AudioService";
protected static final boolean DEBUG_VOL = Log.isLoggable(TAG + ".VOL", Log.DEBUG);

private void adjustStreamVolume(int streamType, int direction, int flags,
        String callingPackage, String caller, int uid) 
......
    if (DEBUG_VOL) Log.d(TAG, "adjustStreamVolume() stream=" + streamType + ", dir=" + direction
                + ", flags=" + flags + ", caller=" + caller);
......

只有在Log.isLoggable返回值为true的时候，adjustStreamVolume()的log才能正常输出。那怎样才能让isLoggable才会返回true呢？

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#定义
isLoggable()在源码中定义如下

/** 
* Checks to see whether or not a log for the specified tag is loggable at the specified level. 
* 
*  The default level of any tag is set to INFO. This means that any level above and including 
*  INFO will be logged. Before you make any calls to a logging method you should check to see 
*  if your tag should be logged. You can change the default level by setting a system property: 
*      'setprop log.tag.<YOUR_LOG_TAG> <LEVEL>' 
*  Where level is either VERBOSE, DEBUG, INFO, WARN, ERROR, ASSERT, or SUPPRESS. SUPPRESS will 
*  turn off all logging for your tag. You can also create a local.prop file that with the 
*  following in it: 
*      'log.tag.<YOUR_LOG_TAG>=<LEVEL>' 
*  and place that in /data/local.prop. 
* 
* @param tag The tag to check. 
* @param level The level to check. 
* @return Whether or not that this is allowed to be logged. 
* @throws IllegalArgumentException is thrown if the tag.length() > 23. 
*/  
public static native boolean isLoggable(String tag, int level);  

从定义可知：

1. isLoggable默认level为Log.INFO；
2. 只有 level >= INFO才能输出，即level >= INFO时isLoggable返回true，反之则返回false；
3. 可以通过setprop log.tag.<YOUR_LOG_TAG> 来改变log的默认level，如adb shell setprop log.tag.AudioService.VOL D。也可以将这些属性按照log.tag.AudioService.VOL = D的形式，写入/data/local.prop中；
4. tag的长度如果超过23个字符则会抛出IllegalArgumentException异常；

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#使用方法

protected static final boolean DEBUG_VOL = Log.isLoggable(TAG + ".VOL", Log.DEBUG);

因为DEBUG是static的变量，所以当Log类被加载时，其值就已经设置好了。如果要使得isLoggable返回为true，那么setprop需要在Log类被加载前设置好，因此使用setprop之后需要重启对应的进程，framework属于每一个进程，那如何重启framework呢？可以使用：

adb shell stop  
adb shell start 

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所以
方法一（机器重启后失效）：
在串口中输入

setprop log.tag.AudioService.VOL D
stop
start

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方法二（机器重启仍生效）：
将log.tag.AudioService.VOL = D加入到/data/local.prop文件中。

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后续发现加个persist关键字，重启也能生效

setprop persist.log.tag.AudioService.VOL D