1. 概述

应用开发与调试中，可以使用Log增加日志打印；然后通过logcat观察&分析日志，定位问题。

2. Log调用

2.1 方法

Log的调用非常简单，通常包括两个步骤：

定义TAG；
Log.e(), w(), i(), d(), v()打印日志

当然，第一步定义TAG不是必须的，但是惯用法。通常一个Activity或Service会定义这个类的一个TAG，从而和其他的组件区分开来；然后在这个组件直接Log(TAG, msg)调用。

2.2 示例

public class MainActivity extends ActionBarActivity {private static final String TAG = "MyActivity";@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);if (savedInstanceState == null) {getSupportFragmentManager().beginTransaction().add(R.id.container, new PlaceholderFragment()).commit();}Log.d(TAG, "onCreate()");}

2.3 源码

Log的源码如下，包括了前面提到的使用方法：

/*** API for sending log output.** <p>Generally, use the Log.v() Log.d() Log.i() Log.w() and Log.e()* methods.** <p>The order in terms of verbosity, from least to most is* ERROR, WARN, INFO, DEBUG, VERBOSE.  Verbose should never be compiled* into an application except during development.  Debug logs are compiled* in but stripped at runtime.  Error, warning and info logs are always kept.** <p><b>Tip:</b> A good convention is to declare a <code>TAG</code> constant* in your class:** <pre>private static final String TAG = "MyActivity";</pre>** and use that in subsequent calls to the log methods.* </p>** <p><b>Tip:</b> Don't forget that when you make a call like* <pre>Log.v(TAG, "index=" + i);</pre>* that when you're building the string to pass into Log.d, the compiler uses a* StringBuilder and at least three allocations occur: the StringBuilder* itself, the buffer, and the String object.  Realistically, there is also* another buffer allocation and copy, and even more pressure on the gc.* That means that if your log message is filtered out, you might be doing* significant work and incurring significant overhead.*/
public final class Log {

2.4 注意事项

Log的调用应该谨慎，不能使用泛滥。一方面，这样不利于定位，大量的无价值的日志会淹没真正有价值的信息，降低定位问题的效率；另一方面，大量的日志会影响程序的运行效率。而且对于手机设备来讲，会影响电池的续航能力（如前面源码中的描述）。

3. logcat

logcat用于捕获（不限于）Log输出的日志信息，包括adb logcat和Eclipse （ADT）的logcat窗口两种方法。

3.1 adb logcat

直接通过adb logcat查看日志，示例：

flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$ ./adb logcat
--------- beginning of /dev/log/system
I/Vold    (   78): Vold 2.1 (the revenge) firing up
D/Vold    (   78): Volume sdcard state changing -1 (Initializing) -> 0 (No-Media)
W/DirectVolume(   78): Kernel block uevent 'PARTN' 1 pendingparts 0
D/Vold    (   78): Volume sdcard state changing 0 (No-Media) -> 2 (Pending)
W/DirectVolume(   78): Kernel block uevent 'PARTN' 1, 1 minor 1
D/Vold    (   78): Volume sdcard state changing 2 (Pending) -> 1 (Idle-Unmounted)
E/NetlinkEvent(   78): NetlinkEvent::FindParam(): Parameter 'SWITCH_NAME' not found
E/NetlinkEvent(   78): NetlinkEvent::FindParam(): Parameter 'SWITCH_STATE' not found
W/Vold    (   78): Switch /devices/virtual/switch/usb_mass_storage event missing name/state info
E/NetlinkEvent(   78): NetlinkEvent::FindParam(): Parameter 'SWITCH_NAME' not found
E/NetlinkEvent(   78): NetlinkEvent::FindParam(): Parameter 'SWITCH_STATE' not found
W/Vold    (   78): Switch /devices/virtual/switch/usb_mass_storage event missing name/state info
W/Vold    (   78): open sys/devices/platform/msm_hsusb/gadget/lun1/file fail:(No such file or directory)
D/VoldCmdListener(   78): volume list
D/VoldCmdListener(   78): share status ums
D/VoldCmdListener(   78): share status ums
D/VoldCmdListener(   78): volume mount /mnt/sdcard
E/DirectVolume(   78): getDeviceNodes i 0 j 0 1
E/Vold    (   78): /dev/block/vold/179:1 being considered for volume sdcard
D/Vold    (   78): Volume sdcard state changing 1 (Idle-Unmounted) -> 3 (Checking)
I/Vold    (   78): Filesystem check completed OK
I/Vold    (   78): Device /dev/block/vold/179:1, target /mnt/sdcard mounted @ /mnt/secure/staging
D/Vold    (   78): Volume sdcard state changing 3 (Checking) -> 4 (Mounted)
D/VoldCmdListener(   78): asec list
E/NetlinkEvent(   78): NetlinkEvent::FindParam(): Parameter 'SWITCH_NAME' not found
E/NetlinkEvent(   78): NetlinkEvent::FindParam(): Parameter 'SWITCH_STATE' not found
W/Vold    (   78): Switch /devices/virtual/switch/usb_mass_storage event missing name/state info
E/NetlinkEvent(   78): NetlinkEvent::FindParam(): Parameter 'SWITCH_NAME' not found

可以看到简单的logcat会输出大量的日志信息。手机厂商通常会预置一些app，消费者拿到手机之后，也会安装一些。由于某些app会滥用Log，因此会导致大量的日志打印出来。

为此，需要对logcat的结果进行过滤。

3.2 adb logcat --help

最简单地，我们直接查阅logcat的帮助信息：

flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$ ./adb logcat --help
Usage: logcat [options] [filterspecs]
options include:-s              Set default filter to silent.Like specifying filterspec '*:s'-f <filename>   Log to file. Default to stdout-r [<kbytes>]   Rotate log every kbytes. (16 if unspecified). Requires -f-n <count>      Sets max number of rotated logs to <count>, default 4-v <format>     Sets the log print format, where <format> is one of:brief process tag thread raw time threadtime long-c              clear (flush) the entire log and exit-d              dump the log and then exit (don't block)-t <count>      print only the most recent <count> lines (implies -d)-g              get the size of the log's ring buffer and exit-b <buffer>     request alternate ring buffer('main' (default), 'radio', 'events')-B              output the log in binary
filterspecs are a series of <tag>[:priority]where <tag> is a log component tag (or * for all) and priority is:V    VerboseD    DebugI    InfoW    WarnE    ErrorF    FatalS    Silent (supress all output)'*' means '*:d' and <tag> by itself means <tag>:vIf not specified on the commandline, filterspec is set from ANDROID_LOG_TAGS.
If no filterspec is found, filter defaults to '*:I'If not specified with -v, format is set from ANDROID_PRINTF_LOG
or defaults to "brief"flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$

或者从Android官网查阅详细信息。由于众所周知的原因，频繁进入developer.android.com是很费劲的，因此最好把Android SDK docs下载一份到本地，具体请参考 Ubuntu下面搭建Android应用开发环境。示例：

3.3 adb logcat使用示例

只显示特定的Activity的debug及以上的日志：

flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$ ./adb logcat MyActivity:d *:S
--------- beginning of /dev/log/system
--------- beginning of /dev/log/main
D/MyActivity( 1939): onCreate()
D/MyActivity( 1939): onResume()
D/MyActivity( 1939): onStop()

如果显示MyActivity的所有日志，则可以直接adb logcat MyActivity *:S

flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$ ./adb logcat MyActivity *:S
--------- beginning of /dev/log/system
--------- beginning of /dev/log/main
D/MyActivity( 1939): onCreate()
D/MyActivity( 1939): onResume()
D/MyActivity( 1939): onStop()

或者

flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$ ./adb logcat -s MyActivity
--------- beginning of /dev/log/system
--------- beginning of /dev/log/main
D/MyActivity( 1939): onCreate()
D/MyActivity( 1939): onResume()
D/MyActivity( 1939): onStop()

如果日志非常多，希望重定向到文件中，事后分析，也是可以的，只需要> mylog即可：

其他几种常用法：

a. 每条日志前面增加时间戳：-v time

flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$ ./adb logcat -s -v time MyActivity
--------- beginning of /dev/log/system
--------- beginning of /dev/log/main
06-05 19:13:57.089 D/MyActivity( 1939): onCreate()
06-05 19:13:57.099 D/MyActivity( 1939): onResume()
06-05 19:14:00.259 D/MyActivity( 1939): onStop()

b. 取完日志之后就退出adb进程：-d

flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$ ./adb logcat -s -v time -d MyActivity
--------- beginning of /dev/log/system
--------- beginning of /dev/log/main
06-05 19:13:57.089 D/MyActivity( 1939): onCreate()
06-05 19:13:57.099 D/MyActivity( 1939): onResume()
06-05 19:14:00.259 D/MyActivity( 1939): onStop()
flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$

c. 日志本身是缓存的，如果要清除历史日志，则使用-c，然后再用前面的命令：

flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$ ./adb logcat -c
flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$ ./adb logcat -s -v time -d MyActivity
--------- beginning of /dev/log/main
flying-bird@flyingbird:~/software/android/adt-bundle-linux-x86-20140321/sdk/platform-tools$ ./adb logcat -s -v time MyActivity
--------- beginning of /dev/log/main
06-05 19:17:07.769 D/MyActivity( 1939): onCreate()
06-05 19:17:07.769 D/MyActivity( 1939): onResume()
06-05 19:17:16.299 D/MyActivity( 1939): onStop()

3.4 Eclipse Logcat View

几种常用的过滤方法：

1. 指定某个Activity的日志：tag:MyActivity

2. 某个app：app:com.example.helloworld

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