Syntax#

• Log.v(String tag, String msg, Throwable tr)
• Log.v(String tag, String msg)
• Log.d(String tag, String msg, Throwable tr)
• Log.d(String tag, String msg)
• Log.i(String tag, String msg, Throwable tr)
• Log.i(String tag, String msg)
• Log.w(String tag, String msg, Throwable tr)
• Log.w(String tag, String msg)
• Log.e(String tag, String msg, Throwable tr)
• Log.e(String tag, String msg)

Parameters#

Logcat is a command-line tool that dumps a log of system messages, including stack traces when the device throws an error and messages that you have written from your app with the Log class.

When to use

If you are considering using Java’s System.out methods for printing to the console instead of using one of Android’s Log methods, then you should know that they basically work the same. However, it is better to avoid using Java’s methods because the extra information and formatting provided by Android’s Log methods are more beneficial. Also, the System.out print methods are redirected to the Log.i() method.

Useful links

• Android developer official documentation for Log and logcat.

• Stackoveflow Question : https://stackoverflow.com/questions/7959263/android-log-v-log-d-log-i-log-w-log-e-when-to-use-each-one

Filtering the logcat output

It is helpful to filter the logcat output because there are many messages which are not of interest. To filter the output, open the “Android Monitor” and click on the drop down on the top-right and select Edit Filter Configuration

Now you can add custom filters to show messages which are of interest, as well as filter out well-known log lines which can safely be ignored. To ignore a part of the output you may define a Regular Expression. Here is an example of excluding matching tags:

^(?!(HideMe|AndThis))

This can be entered by following this example:

The above is a regular expression which excludes inputs. If you wanted to add another tag to the blacklist, add it after a pipe | character. For example, if you wanted to blacklist “GC”, you would use a filter like this:

^(?!(HideMe|AndThis|GC))

For more documentation and examples visit Logging and using Logcat

Logging

Any quality Android application will keep track of what it’s doing through application logs. These logs allow easy debugging help for the developer to diagnose what’s going on with the application. Full Android Documentation can be found here, but a summary follows:

Basic Logging

Log.v(String tag, String msg);

And the Android system will write a message to the logcat:

07-28 12:00:00.759 24812-24839/my.packagename V/MyAnimator: Some log messages
 └ time stamp             |  app.package┘     |    └ any tag  |
     process & thread ids ┘          log level┘               └ the log message
 

TIP:
Notice the process id and the thread id. If they are the same - the log is coming from the main/UI thread!

Any tag can be used, but it is common to use the class name as a tag:

public static final String tag = MyAnimator.class.getSimpleName();

Log Levels

Motivation For Logging

E/MyApplication: Process: com.example.myapplication, PID: 25788
                          com.example.SomeRandomException: Expected string, got 'null' instead

Followed by a bunch of stack traces that would eventually lead to the offending line, where stepping through with a debugger would eventually lead to the problem

However, the log trace of an application with logging enabled could look something like this:

V/MyApplication: Looking for file myFile.txt on the SD card
D/MyApplication: Found file myFile.txt at path <path>
V/MyApplication: Opening file myFile.txt
D/MyApplication: Finished reading myFile.txt, found 0 lines
V/MyApplication: Closing file myFile.txt
...
E/MyApplication: Process: com.example.myapplication, PID: 25788
                          com.example.SomeRandomException: Expected string, got 'null' instead

A quick glance at the logs and it is obvious that the file was empty.

Things To Considering When Logging:

Log Readability:

Performance:

Security:

Conclusion:

For more documentation and examples visit Logging and using Logcat

Log with link to source directly from Logcat

This is a nice trick to add a link to code, so it will be easy to jump to the code that issued the log.

With the following code, this call:

MyLogger.logWithLink("MyTag","param="+param);

Will result in:

07-26...012/com.myapp D/MyTag: MyFrag:onStart(param=3)  (MyFrag.java:2366) // << logcat converts this to a link to source!

This is the code (inside a class called MyLogger):

static StringBuilder sb0 = new StringBuilder(); // reusable string object

public static void logWithLink(String TAG, Object param) {
    StackTraceElement stack = Thread.currentThread().getStackTrace()[3];
    sb0.setLength(0);
    String c = stack.getFileName().substring(0, stack.getFileName().length() - 5); // removes the ".java"
    sb0.append(c).append(":");
    sb0.append(stack.getMethodName()).append('(');
    if (param != null) {
        sb0.append(param);
    }
    sb0.append(") ");
    sb0.append(" (").append(stack.getFileName()).append(':').append(stack.getLineNumber()).append(')');
    Log.d(TAG, sb0.toString());
}

This is a basic example, it can be easily extended to issue a link to the caller (hint: the stack will be 4 instead of 3), and you can also add other relevant information.

Using the Logcat

Logcat is a command-line tool that dumps a log of system messages, including stack traces when the device throws an error and messages that you have written from your app with the Log class.

The Logcat output can be displayed within Android Studio’s Android Monitor or with adb command line.

In Android Studio

Show by clicking the “Android Monitor” icon: Or by pressing Alt+6 on Windows/Linux or CMD+6 on Mac.

via command line:

Simple usage:

$ adb logcat

With timestamps:

$ adb logcat -v time

Filter on specific text:

$ adb logcat -v time | grep 'searchtext'

There are many options and filters available to command line logcat, documented here.
A simple but useful example is the following filter expression that displays all log messages with priority level “error”, on all tags:

$ adb logcat *:E

Generating Logging code

Android Studio’s Live templates can offer quite a few shortcuts for quick logging.
To use Live templates, all you need to do is to start typing the template name, and hit TAB or enter to insert the statement.

Examples:

• logi → turns into → android.util.Log.i(TAG, "$METHOD_NAME$: $content$");
  • $METHOD_NAME$ will automatically be replaced with your method name, and the cursor will wait for the content to be filled.
• loge → same, for error
• etc. for the rest of the logging levels.

Full list of templates can be found in Android Studio’s settings (ALT+s and type “live”). And it is possible to add your custom templates as well.

If you find Android Studio’s Live templates not enough for your needs, you can consider Android Postfix Plugin

This is a very useful library which helps you to avoid writing the logging line manually.

The syntax is absolutely simple:

.log - Logging. If there is constant variable “TAG”, it use “TAG” . Else it use class name.

Android Studio usage

1. Hide/show printed information:

2. Control verbosity of the logging:

3. Disable/enable opening log window when starting run/debug application

Clear logs