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Pengaturcaraan

1. Gambaran keseluruhan

Logback adalah salah satu kerangka pembalakan yang paling banyak digunakan di Komuniti Java. Ini pengganti pendahulunya, Log4j. Logback menawarkan pelaksanaan yang lebih pantas daripada Log4j, memberikan lebih banyak pilihan untuk konfigurasi, dan lebih banyak fleksibiliti dalam mengarkibkan fail log lama.

Pengenalan ini akan memperkenalkan seni bina Logback dan menunjukkan kepada anda bagaimana anda dapat menggunakannya menjadikan aplikasi anda lebih baik.

2. Seni Bina Logback

Tiga kelas merangkumi seni bina Logback; Logger , Appender , dan Layout .

Logger adalah konteks untuk mesej log. Ini adalah kelas yang berinteraksi dengan aplikasi untuk membuat mesej log.

Appenders meletakkan mesej log di destinasi akhir mereka. Seorang Logger boleh mempunyai lebih dari satu Appender. Kami biasanya menganggap Appenders dilampirkan pada fail teks, tetapi Logback jauh lebih kuat daripada itu.

Susun atur menyiapkan mesej untuk dikeluarkan. Logback menyokong penciptaan kelas khusus untuk memformat mesej, serta pilihan konfigurasi yang mantap untuk yang ada.

3. Persediaan

3.1. Ketergantungan Maven

Logback menggunakan Simple Logging Facade untuk Java (SLF4J) sebagai antara muka asalnya. Sebelum kita dapat memulakan pembalakan mesej, kita perlu menambahkan Logback dan Slf4j ke pom.xml kami :

 ch.qos.logback logback-core 1.2.3   org.slf4j slf4j-api 1.7.30 test

Maven Central mempunyai versi terbaru dari Logback Core dan versi terbaru dari slf4j-api .

3.2. Laluan Kelas

Logback juga memerlukan logback-classic.jar di classpath untuk runtime.

Kami akan menambahkannya ke pom.xml sebagai pergantungan ujian:

 ch.qos.logback logback-classic 1.2.3

4. Contoh dan Konfigurasi Asas

Mari kita mulakan dengan contoh ringkas menggunakan Logback dalam aplikasi.

Pertama, kita memerlukan fail konfigurasi. Kami akan membuat fail teks bernama logback.xml dan meletakkannya di suatu tempat di classpath kami:

   %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n

Seterusnya, kita memerlukan kelas sederhana dengan kaedah utama :

public class Example { private static final Logger logger = LoggerFactory.getLogger(Example.class); public static void main(String[] args) { logger.info("Example log from {}", Example.class.getSimpleName()); } }

Kelas ini membuat maklumat Logger dan panggilan () untuk menghasilkan mesej log.

Semasa kita menjalankan Contoh kita melihat mesej kita masuk ke konsol:

20:34:22.136 [main] INFO Example - Example log from Example

Sangat mudah untuk melihat mengapa Logback sangat popular; kita akan berjalan dalam beberapa minit.

Konfigurasi dan kod ini memberi kita beberapa petunjuk bagaimana ini berfungsi.

  1. Kami mempunyai pelamar bernama STDOUT yang merujuk nama kelas ConsoleAppender.
  2. Terdapat corak yang menerangkan format mesej log kami.
  3. Kod kami mencipta Logger dan kami menyampaikan mesej kita kepadanya melalui info () kaedah .

Sekarang setelah kita memahami asasnya, mari kita lihat lebih dekat.

5. Kontek Logger

5.1. Membuat Konteks

Untuk memasukkan mesej ke Logback, kami memulakan Logger dari SLF4J atau Logback:

private static final Logger logger = LoggerFactory.getLogger(Example.class);

Dan kemudian menggunakannya:

logger.info("Example log from {}", Example.class.getSimpleName());

Ini adalah konteks pembalakan kami. Semasa kami membuatnya, kami lulus kelas LoggerFactory . Ini memberikan nama kepada Pembalak (terdapat juga kelebihan beban yang menerima String).

Konteks pembalakan wujud dalam hierarki yang sangat mirip dengan hierarki objek Java:

  1. Pembalak adalah nenek moyang ketika namanya, diikuti dengan titik, awalan nama penebang keturunan
  2. Pembalak adalah ibu bapa apabila tidak ada nenek moyang antara mereka dan anak

Sebagai contoh, kelas Contoh di bawah terdapat dalam pakej com.baeldung.logback . Terdapat satu lagi kelas bernama ExampleAppender dalam pakej com.baeldung.logback.appenders .

Contoh App Logger adalah anak dari Contoh Logger.

Semua pembalak adalah keturunan dari penebang akar yang telah ditentukan.

A Logger mempunyai Level, yang boleh ditetapkan sama ada melalui konfigurasi atau dengan Logger.setLevel (). Menetapkan tahap dalam kod mengatasi fail konfigurasi.

Tahap yang mungkin adalah, mengikut urutan keutamaan: TRACE, DEBUG, INFO, WARN dan ERROR.Setiap tingkat mempunyai kaedah yang sesuai yang kami gunakan untuk log mesej pada tahap itu.

Sekiranya seorang Pembalak tidak diberi peringkat secara eksplisit, ia akan mewarisi tahap nenek moyang terdekatnya. Logger root lalai ke DEBUG. Kami akan melihat cara mengatasi ini di bawah.

5.2. Menggunakan Konteks

Mari buat contoh program yang menunjukkan penggunaan konteks dalam hierarki pembalakan:

ch.qos.logback.classic.Logger parentLogger = (ch.qos.logback.classic.Logger) LoggerFactory.getLogger("com.baeldung.logback"); parentLogger.setLevel(Level.INFO); Logger childlogger = (ch.qos.logback.classic.Logger)LoggerFactory.getLogger("com.baeldung.logback.tests"); parentLogger.warn("This message is logged because WARN > INFO."); parentLogger.debug("This message is not logged because DEBUG < INFO."); childlogger.info("INFO == INFO"); childlogger.debug("DEBUG < INFO");

When we run this, we see these messages:

20:31:29.586 [main] WARN com.baeldung.logback - This message is logged because WARN > INFO. 20:31:29.594 [main] INFO com.baeldung.logback.tests - INFO == INFO

We start by retrieving a Logger named com.baeldung.logback and cast it to a ch.qos.logback.classic.Logger.

A Logback context is needed to set the level in the next statement; note that the SLF4J's abstract logger does not implement setLevel().

We set the level of our context to INFO;we then create another logger named com.baeldung.logback.tests.

We log two messages with each context to demonstrate the hierarchy. Logback logs the WARN, and INFO messages and filters the DEBUGmessages.

Now, let's use the root logger:

ch.qos.logback.classic.Logger logger = (ch.qos.logback.classic.Logger)LoggerFactory.getLogger("com.baeldung.logback"); logger.debug("Hi there!"); Logger rootLogger = (ch.qos.logback.classic.Logger)LoggerFactory.getLogger(org.slf4j.Logger.ROOT_LOGGER_NAME); logger.debug("This message is logged because DEBUG == DEBUG."); rootLogger.setLevel(Level.ERROR); logger.warn("This message is not logged because WARN < ERROR."); logger.error("This is logged.");

We see these messages when we execute this snippet:

20:44:44.241 [main] DEBUG com.baeldung.logback - Hi there! 20:44:44.243 [main] DEBUG com.baeldung.logback - This message is logged because DEBUG == DEBUG. 20:44:44.243 [main] ERROR com.baeldung.logback - This is logged.

To conclude, we started with a Logger context and printed a DEBUG message.

We then retrieved the root logger using its statically defined name and set its level to ERROR.

And finally, we demonstrated that Logback actually does filter any statement less than an error.

5.3. Parameterized Messages

Unlike the messages in the sample snippets above, most useful log messages required appending Strings. This entails allocating memory, serializing objects, concatenating Strings, and potentially cleaning up the garbage later.

Consider the following message:

log.debug("Current count is " + count);

We incur the cost of building the message whether the Logger logs the message or not.

Logback offers an alternative with its parameterized messages:

log.debug("Current count is {}", count);

The braces {} will accept any Object and uses its toString() method to build a message only after verifying that the log message is required.

Let's try some different parameters:

String message = "This is a String"; Integer zero = 0; try { logger.debug("Logging message: {}", message); logger.debug("Going to divide {} by {}", 42, zero); int result = 42 / zero; } catch (Exception e) { logger.error("Error dividing {} by {} ", 42, zero, e); }

This snippet yields:

21:32:10.311 [main] DEBUG com.baeldung.logback.LogbackTests - Logging message: This is a String 21:32:10.316 [main] DEBUG com.baeldung.logback.LogbackTests - Going to divide 42 by 0 21:32:10.316 [main] ERROR com.baeldung.logback.LogbackTests - Error dividing 42 by 0 java.lang.ArithmeticException: / by zero at com.baeldung.logback.LogbackTests.givenParameters_ValuesLogged(LogbackTests.java:64) ...

We see how a String, an int, and an Integer can be passed in as parameters.

Also, when an Exception is passed as the last argument to a logging method, Logback will print the stack trace for us.

6. Detailed Configuration

In the previous examples, we were using the 11-line configuration file to we created in section 4 to print log messages to the console. This is Logback's default behavior; if it cannot find a configuration file, it creates a ConsoleAppender and associates it with the root logger.

6.1. Locating Configuration Information

A configuration file can be placed in the classpath and named either logback.xml or logback-test.xml.

Here's how Logback will attempt to find configuration data:

  1. Search for files named logback-test.xml, logback.groovy,or logback.xml in the classpath, in that order.
  2. If the library does not find those files, it will attempt to use Java's ServiceLoader to locate an implementor of the com.qos.logback.classic.spi.Configurator.
  3. Configure itself to log output directly to the console

Note: the current version of Logback does not support Groovy configuration due to there not being a version of Groovy compatible with Java 9.

6.2. Basic Configuration

Let's take a closer look at our example configuration.

The entire file is in tags.

We see a tag that declares an Appender of type ConsoleAppender, and names it STDOUT. Nested within that tag is an encoder. It has a pattern with what looks like sprintf-style escape codes:

  %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n

Last, we see a root tag. This tag sets the root logger to DEBUG mode and associates its output with the Appender named STDOUT:

6.3. Troubleshooting Configuration

Logback configuration files can get complicated, so there are several built-in mechanisms for troubleshooting.

To see debug information as Logback processes the configuration, you can turn on debug logging:

 ...

Logback will print status information to the console as it processes the configuration:

23:54:23,040 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Found resource [logback-test.xml] at [file:/Users/egoebelbecker/ideaProjects/logback-guide/out/test/resources/logback-test.xml] 23:54:23,230 |-INFO in ch.qos.logback.core.joran.action.AppenderAction - About to instantiate appender of type [ch.qos.logback.core.ConsoleAppender] 23:54:23,236 |-INFO in ch.qos.logback.core.joran.action.AppenderAction - Naming appender as [STDOUT] 23:54:23,247 |-INFO in ch.qos.logback.core.joran.action.NestedComplexPropertyIA - Assuming default type [ch.qos.logback.classic.encoder.PatternLayoutEncoder] for [encoder] property 23:54:23,308 |-INFO in ch.qos.logback.classic.joran.action.RootLoggerAction - Setting level of ROOT logger to DEBUG 23:54:23,309 |-INFO in ch.qos.logback.core.joran.action.AppenderRefAction - Attaching appender named [STDOUT] to Logger[ROOT] 23:54:23,310 |-INFO in ch.qos.logback.classic.joran.action.ConfigurationAction - End of configuration. 23:54:23,313 |-INFO in [email protected] - Registering current configuration as safe fallback point

If warnings or errors are encountered while parsing the configuration file, Logback writes status messages to the console.

There is a second mechanism for printing status information:

  ...

The StatusListener intercepts status messages and prints them during configuration and while the program is running.

The output from all configuration files is printed, making it useful for locating “rogue” configuration files on the classpath.

6.4. Reloading Configuration Automatically

Reloading logging configuration while an application is running is a powerful troubleshooting tool. Logback makes this possible with the scan parameter:

 ...

The default behavior is to scan the configuration file for changes every 60 seconds. Modify this interval by adding scanPeriod:

 ...

We can specify values in milliseconds, seconds, minutes, or hours.

6.5. Modifying Loggers

In our sample file above we set the level of the root logger and associated it with the console Appender.

We can set the level for any logger:

   %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n

Let's add this to our classpath and run this code:

Logger foobar = (ch.qos.logback.classic.Logger) LoggerFactory.getLogger("com.baeldung.foobar"); Logger logger = (ch.qos.logback.classic.Logger) LoggerFactory.getLogger("com.baeldung.logback"); Logger testslogger = (ch.qos.logback.classic.Logger) LoggerFactory.getLogger("com.baeldung.logback.tests"); foobar.debug("This is logged from foobar"); logger.debug("This is not logged from logger"); logger.info("This is logged from logger"); testslogger.info("This is not logged from tests"); testslogger.warn("This is logged from tests");

We see this output:

00:29:51.787 [main] DEBUG com.baeldung.foobar - This is logged from foobar 00:29:51.789 [main] INFO com.baeldung.logback - This is logged from logger 00:29:51.789 [main] WARN com.baeldung.logback.tests - This is logged from tests

By not setting the level of our Loggers programmatically, the configuration sets them; com.baeldung.foobar inherits DEBUG from the root logger.

Loggersalso inherit the appender-ref from the root logger. As we'll see below, we can override this.

6.6. Variable Substitution

Logback configuration files support variables. We define variables inside the configuration script or externally. A variable can be specified at any point in a configuration script in place of a value.

For example, here is the configuration for a FileAppender:

  ${LOG_DIR}/tests.log true  %-4relative [%thread] %-5level %logger{35} - %msg%n

At the top of the configuration, we declared a propertynamed LOG_DIR.Then we used it as part of the path to the file inside the appender definition.

Properties are declared in a tag in configuration scripts. But they are also available from outside sources, such as system properties. We could omit the property declaration in this example and set the value of LOG_DIR on the command line:

$ java -DLOG_DIR=/var/log/application com.baeldung.logback.LogbackTests

We specify the value of the property with ${propertyname}. Logback implements variables as text replacement. Variable substitution can occur at any point in a configuration file where a value can be specified.

7. Appenders

Loggers pass LoggingEvents to Appenders.Appenders do the actual work of logging. We usually think of logging as something that goes to a file or the console, but Logback is capable of much more. Logback-core provides several useful appenders.

7.1. ConsoleAppender

We've seen ConsoleAppenderin action already. Despite its name, ConsoleAppender appends messages to System.outor System.err.

It uses an OutputStreamWriter to buffer the I/O, so directing it to System.err does not result in unbuffered writing.

7.2. FileAppender

FileAppenderappends messages to a file. It supports a broad range of configuration parameters. Let's add a file appender to our basic configuration:

    %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n    tests.log true  %-4relative [%thread] %-5level %logger{35} - %msg%n

The FileAppender is configured with a file name via . The tag instructs the Appenderto append to an existing file rather than truncating it. If we run the test several times, we see that the logging output is appended to the same file.

If we re-run our test from above, messages from com.baeldung.logback.tests go to both the console and to a file named tests.log. The descendant logger inherits the root logger's association with the ConsoleAppender with its association with FileAppender. Appenders are cumulative.

We can override this behavior:

Setting additivity to falsedisables the default behavior. Tests will not log to the console, and neither will any of its descendants.

7.3. RollingFileAppender

Oftentimes, appending log messages to the same file is not the behavior we need. We want files to “roll” based on time, log file size, or a combination of both.

For this, we have RollingFileAppender:

  ${LOG_FILE}.log   ${LOG_FILE}.%d{yyyy-MM-dd}.gz  30 3GB   %-4relative [%thread] %-5level %logger{35} - %msg%n

A RollingFileAppenderhas a RollingPolicy.In this sample configuration, we see a TimeBasedRollingPolicy.

Similar to the FileAppender, we configured this appender with a file name. We declared a property and used it for this because we'll be reusing the file name below.

We define a fileNamePattern inside the RollingPolicy.This pattern defines not just the name of files, but also how often to roll them. TimeBasedRollingPolicyexamines the pattern and rolls at the most finely defined period.

For example:

   ${LOG_DIR}/${LOG_FILE}.log  ${LOG_DIR}/%d{yyyy/MM}/${LOG_FILE}.gz 3GB

The active log file is /var/logs/application/LogFile.This file rolls over at the beginning of each month into /Current Year/Current Month/LogFile.gzand RollingFileAppender createsa new active file.

When the total size of archived files reaches 3GB, RollingFileAppenderdeletes archives on a first-in-first-out basis.

There are codes for a week, hour, minute, second, and even millisecond. Logback has a reference here.

RollingFileAppenderalso has built-in support for compressing files. It compresses our rolled files because named our them LogFile.gz.

TimeBasedPolicyisn't our only option for rolling files. Logback also offers SizeAndTimeBasedRollingPolicy,which will roll based on current log file size as well as time. It also offers a FixedWindowRollingPolicywhich rolls log file names each time the logger is started.

We can also write our own RollingPolicy.

7.4. Custom Appenders

We can create custom appenders by extending one of Logback's base appender classes. We have a tutorial for creating custom appenders here.

8. Layouts

Layouts format log messages. Like the rest of Logback, Layoutsare extensible and we can create our own. However, the default PatternLayout offers what most applications need and then some.

We've used PatternLayout in all of our examples so far:

 %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n

This configuration script contains the configuration for PatternLayoutEncoder.We pass an Encoder to our Appender,and this encoder uses the PatternLayout to format the messages.

The text in the tag defines how log messages are formatting. PatternLayout implements a large variety of conversion words and format modifiers for creating patterns.

Let's break this one down. PatternLayout recognizes conversion words with a %, so the conversions in our pattern generate:

  • %d{HH:mm:ss.SSS} – a timestamp with hours, minutes, seconds and milliseconds
  • [%thread] – the thread name generating the log message, surrounded by square brackets
  • %-5level – the level of the logging event, padded to 5 characters
  • %logger{36} – the name of the logger, truncated to 35 characters
  • %msg%n – the log messages followed by the platform dependent line separator character

So we see messages similar to this:

21:32:10.311 [main] DEBUG com.baeldung.logback.LogbackTests - Logging message: This is a String

An exhaustive list of conversion words and format modifiers can be found here.

9. Conclusion

In this extensive guide, we covered the fundamentals of using Logback in an application.

Kami melihat tiga komponen utama dalam seni bina Logback: pembalak, pelengkap, dan susun atur. Logback's mempunyai skrip konfigurasi yang kuat, yang kami gunakan untuk memanipulasi komponen untuk menyaring dan memformat mesej. Kami juga melihat dua lampiran fail yang paling biasa digunakan untuk membuat, menggulung, mengatur dan memampatkan fail log.

Seperti biasa, coretan kod boleh didapati di GitHub.