Log

Kotlin | Java

public final class Log

extends Object

API for sending log output.

Generally, you should use the Log.v() , Log.d() , Log.i() , Log.w() , and Log.e() methods to write logs. You can then view the logs in logcat.

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.

Tip: A good convention is to declare a TAG constant in your class:

private static final String TAG = "MyActivity";

and use that in subsequent calls to the log methods.

Tip: Don't forget that when you make a call like

Log.v(TAG, "index=" + i);

Summary

Constants int ASSERT Priority constant for the println method. int DEBUG Priority constant for the println method; use Log.d. int ERROR Priority constant for the println method; use Log.e. int INFO Priority constant for the println method; use Log.i. int VERBOSE Priority constant for the println method; use Log.v. int WARN Priority constant for the println method; use Log.w.

Public methods static int d(String tag, String msg, Throwable tr) Send a DEBUG log message and log the exception. static int d(String tag, String msg) Send a DEBUG log message. static int e(String tag, String msg) Send an ERROR log message. static int e(String tag, String msg, Throwable tr) Send a ERROR log message and log the exception. static String getStackTraceString(Throwable tr) Handy function to get a loggable stack trace from a Throwable static int i(String tag, String msg, Throwable tr) Send a INFO log message and log the exception. static int i(String tag, String msg) Send an INFO log message. static boolean isLoggable(String tag, int level) Checks to see whether or not a log for the specified tag is loggable at the specified level. static int println(int priority, String tag, String msg) Low-level logging call. static int v(String tag, String msg) Send a VERBOSE log message. static int v(String tag, String msg, Throwable tr) Send a VERBOSE log message and log the exception. static int w(String tag, Throwable tr) Send a WARN log message and log the exception. static int w(String tag, String msg, Throwable tr) Send a WARN log message and log the exception. static int w(String tag, String msg) Send a WARN log message. static int wtf(String tag, String msg) What a Terrible Failure: Report a condition that should never happen. static int wtf(String tag, Throwable tr) What a Terrible Failure: Report an exception that should never happen. static int wtf(String tag, String msg, Throwable tr) What a Terrible Failure: Report an exception that should never happen.

Inherited methods java.lang.Object From class Object clone() Creates and returns a copy of this object. boolean equals(Object obj) Indicates whether some other object is "equal to" this one. void finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. final Class<?> getClass() Returns the runtime class of this Object . int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long timeout, int nanos) Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed. final void wait(long timeout) Causes the current thread to wait until either another thread invokes the notify() method or the notifyAll() method for this object, or a specified amount of time has elapsed. final void wait() Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object.

Constants

ASSERT Added in API level 1 public static final int ASSERT Priority constant for the println method. Constant Value: 7 (0x00000007)

DEBUG Added in API level 1 public static final int DEBUG Priority constant for the println method; use Log.d. Constant Value: 3 (0x00000003)

ERROR Added in API level 1 public static final int ERROR Priority constant for the println method; use Log.e. Constant Value: 6 (0x00000006)

INFO Added in API level 1 public static final int INFO Priority constant for the println method; use Log.i. Constant Value: 4 (0x00000004)

VERBOSE Added in API level 1 public static final int VERBOSE Priority constant for the println method; use Log.v. Constant Value: 2 (0x00000002)

WARN Added in API level 1 public static final int WARN Priority constant for the println method; use Log.w. Constant Value: 5 (0x00000005)

Public methods

d Added in API level 1 public static int d (String tag, String msg, Throwable tr) Send a DEBUG log message and log the exception. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value may be null . tr Throwable : An exception to log This value may be null . Returns int

d Added in API level 1 public static int d (String tag, String msg) Send a DEBUG log message. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value cannot be null . Returns int

e Added in API level 1 public static int e (String tag, String msg) Send an ERROR log message. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value cannot be null . Returns int

e Added in API level 1 public static int e (String tag, String msg, Throwable tr) Send a ERROR log message and log the exception. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value may be null . tr Throwable : An exception to log This value may be null . Returns int

getStackTraceString Added in API level 1 public static String getStackTraceString (Throwable tr) Handy function to get a loggable stack trace from a Throwable Parameters tr Throwable : An exception to log This value may be null . Returns String This value cannot be null .

i Added in API level 1 public static int i (String tag, String msg, Throwable tr) Send a INFO log message and log the exception. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value may be null . tr Throwable : An exception to log This value may be null . Returns int

i Added in API level 1 public static int i (String tag, String msg) Send an INFO log message. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value cannot be null . Returns int

isLoggable Added in API level 1 public static boolean isLoggable (String tag, int level) 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, or ASSERT. 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. Parameters tag String : The tag to check. This value may be null . level int : The level to check. Value is ASSERT , ERROR , WARN , INFO , DEBUG , or VERBOSE Returns boolean Whether or not that this is allowed to be logged. Throws IllegalArgumentException is thrown if the tag.length() > 23 for Nougat (7.0) releases (API <= 23) and prior, there is no tag limit of concern after this API level.

println Added in API level 1 public static int println (int priority, String tag, String msg) Low-level logging call. Parameters priority int : The priority/type of this log message Value is ASSERT , ERROR , WARN , INFO , DEBUG , or VERBOSE tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value cannot be null . Returns int The number of bytes written.

v Added in API level 1 public static int v (String tag, String msg) Send a VERBOSE log message. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value cannot be null . Returns int

v Added in API level 1 public static int v (String tag, String msg, Throwable tr) Send a VERBOSE log message and log the exception. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value may be null . tr Throwable : An exception to log This value may be null . Returns int

w Added in API level 1 public static int w (String tag, Throwable tr) Send a WARN log message and log the exception. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . tr Throwable : An exception to log This value may be null . Returns int

w Added in API level 1 public static int w (String tag, String msg, Throwable tr) Send a WARN log message and log the exception. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value may be null . tr Throwable : An exception to log This value may be null . Returns int

w Added in API level 1 public static int w (String tag, String msg) Send a WARN log message. Parameters tag String : Used to identify the source of a log message. It usually identifies the class or activity where the log call occurs. This value may be null . msg String : The message you would like logged. This value cannot be null . Returns int

wtf Added in API level 8 public static int wtf (String tag, String msg) What a Terrible Failure: Report a condition that should never happen. The error will always be logged at level ASSERT with the call stack. Depending on system configuration, a report may be added to the DropBoxManager and/or the process may be terminated immediately with an error dialog. Parameters tag String : Used to identify the source of a log message. This value may be null . msg String : The message you would like logged. This value may be null . Returns int

wtf Added in API level 8 public static int wtf (String tag, Throwable tr) What a Terrible Failure: Report an exception that should never happen. Similar to wtf(java.lang.String, java.lang.String) , with an exception to log. Parameters tag String : Used to identify the source of a log message. This value may be null . tr Throwable : An exception to log. This value cannot be null . Returns int

wtf Added in API level 8 public static int wtf (String tag, String msg, Throwable tr) What a Terrible Failure: Report an exception that should never happen. Similar to wtf(java.lang.String, java.lang.Throwable) , with a message as well. Parameters tag String : Used to identify the source of a log message. This value may be null . msg String : The message you would like logged. This value may be null . tr Throwable : An exception to log. May be null. This value may be null . Returns int

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.