Java : Handler (logging) with Examples

Handler (Java SE 19 & JDK 19) API Examples.
You will find code examples on most Handler methods.

Summary

A Handler object takes log messages from a Logger and exports them. It might for example, write them to a console or write them to a file, or send them to a network logging service, or forward them to an OS log, or whatever.

Class diagram

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);
logger.setLevel(Level.ALL);

final Handler handler = new ConsoleHandler();
logger.addHandler(handler);

logger.info("TEST LOG!");

// Result (Console)
// ↓
//Jan 07, 2023 10:54:22 PM com.example.logging.HandlerTest$1 method
//INFO: TEST LOG!

Constructors

Handler ()

Default constructor.

class SampleHandler extends Handler {
    @Override
    public void publish(LogRecord record) {
        // Implement ...
    }

    @Override
    public void flush() {
        // Implement ...
    }

    @Override
    public void close() throws SecurityException {
        // Implement ...
    }
}

final Handler handler = new SampleHandler();

System.out.println(handler.getLevel()); // ALL
System.out.println(handler.getFilter()); // null
System.out.println(handler.getFormatter()); // null

Methods

abstract void close ()

Close the Handler and free all associated resources.

final var pattern = Path.of("R:", "java-work", "log.txt");
System.out.println(pattern); // R:\java-work\log.txt

final Handler handler = new FileHandler(pattern.toString());

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);
logger.addHandler(handler);

logger.info("TEST LOG!");

logger.removeHandler(handler);
handler.close();

System.out.println(Files.readString(pattern));

// Result
// ↓
//<?xml version="1.0" encoding="UTF-8" standalone="no"?>
//<!DOCTYPE log SYSTEM "logger.dtd">
//<log>
//<record>
//  <date>2022-03-13T10:43:28.380133800Z</date>
//  <millis>1647168208380</millis>
//  <nanos>133800</nanos>
//  <sequence>1</sequence>
//  <logger>com.example.logging</logger>
//  <level>INFO</level>
//  <class>com.example.logging.HandlerTest$1</class>
//  <method>method</method>
//  <thread>1</thread>
//  <message>TEST LOG!</message>
//</record>
//</log>

abstract void flush ()

Flush any buffered output.

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);

final var outputStream = new ByteArrayOutputStream();
final Handler handler = new StreamHandler(outputStream, new SimpleFormatter());

logger.addHandler(handler);

logger.info("TEST LOG!");

System.out.println("-- before flush --");
System.out.println(outputStream);

handler.flush();

System.out.println("-- after flush --");
System.out.println(outputStream);

// Result
// ↓
//-- before flush --
//
//-- after flush --
//Mar 13, 2022 4:41:49 AM com.example.logging.HandlerTest$2 method
//INFO: TEST LOG!

String getEncoding ()

Return the character encoding for this Handler.

System.out.println(Charset.defaultCharset()); // UTF-8

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);

final Handler handler = new ConsoleHandler();
System.out.println(handler.getEncoding()); // null
logger.addHandler(handler);

logger.info("TEST LOG : 〇△□×");

// An example of outputting to the console for UTF-8.

// Result
// ↓
//Mar 13, 2022 4:46:10 AM com.example.logging.HandlerTest$3 method
//INFO: TEST LOG : 〇△□×

handler.setEncoding("Shift_JIS");
System.out.println(handler.getEncoding()); // Shift_JIS

logger.info("TEST LOG : 〇△□×");

// Result (Garbled characters)
// ↓
//Mar 13, 2022 4:46:10 AM com.example.logging.HandlerTest$3 method
//INFO: TEST LOG : �Z�����~

ErrorManager getErrorManager ()

Retrieves the ErrorManager for this Handler.

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);

final Handler handler = new ConsoleHandler() {
    @Override
    public void publish(LogRecord record) {
        // Intentionally reports an exception.
        reportError("TEST ERROR!",
                new IllegalStateException("TEST EXCEPTION!"),
                ErrorManager.GENERIC_FAILURE);
    }
};

logger.addHandler(handler);

class CustomErrorManager extends ErrorManager {
    @Override
    public synchronized void error(String msg, Exception ex, int code) {
        System.err.println("CUSTOM ERROR! : " + msg + " : " + ex.getMessage());
    }
}

System.out.println(handler.getErrorManager() instanceof CustomErrorManager); // false

logger.info("TEST LOG!");

// Result
// ↓
//java.util.logging.ErrorManager: 0: TEST ERROR!
//java.lang.IllegalStateException: TEST EXCEPTION!
//	at com.example.logging.HandlerTest$6$1.publish(HandlerTest.java:354)
//...

handler.setErrorManager(new CustomErrorManager());
System.out.println(handler.getErrorManager() instanceof CustomErrorManager); // true

logger.info("TEST LOG!");

// Result
// ↓
//CUSTOM ERROR! : TEST ERROR! : TEST EXCEPTION!

Filter getFilter ()

Get the current Filter for this Handler.

final Handler handler = new ConsoleHandler();

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);
logger.addHandler(handler);

System.out.println(handler.getFilter()); // null

logger.info("abcd");
logger.info("EFG");
logger.info("xyz");

// Result
// ↓
//Mar 13, 2022 4:55:38 AM com.example.logging.HandlerTest$5 method
//INFO: abcd
//Mar 13, 2022 4:55:39 AM com.example.logging.HandlerTest$5 method
//INFO: EFG
//Mar 13, 2022 4:55:39 AM com.example.logging.HandlerTest$5 method
//INFO: xyz

final var filter = new Filter() {
    @Override
    public boolean isLoggable(LogRecord record) {
        final var msg = record.getMessage();
        return msg.equals(msg.toLowerCase());
    }
};

handler.setFilter(filter);
System.out.println(filter == handler.getFilter()); // true

logger.info("abcd");
logger.info("EFG");
logger.info("xyz");

// Result
// ↓
//Mar 13, 2022 4:55:39 AM com.example.logging.HandlerTest$5 method
//INFO: abcd
//Mar 13, 2022 4:55:39 AM com.example.logging.HandlerTest$5 method
//INFO: xyz

Formatter getFormatter ()

Return the Formatter for this Handler.

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);

// ConsoleHandler has SimpleFormatter by default.
final Handler handler = new ConsoleHandler();
logger.addHandler(handler);

// java.util.logging.SimpleFormatter@6a8658ff
System.out.println(handler.getFormatter());

logger.info("TEST LOG!");

// Result
// ↓
//Mar 13, 2022 5:00:26 AM com.example.logging.HandlerTest$6 method
//INFO: TEST LOG!

handler.setFormatter(new XMLFormatter());

// java.util.logging.XMLFormatter@491b9b8
System.out.println(handler.getFormatter());

logger.info("TEST LOG!");

// Result
// ↓
//<record>
//  <date>2022-03-13T12:00:26.414943800Z</date>
//  <millis>1647172826414</millis>
//  <nanos>943800</nanos>
//  <sequence>2</sequence>
//  <logger>com.example.logging</logger>
//  <level>INFO</level>
//  <class>com.example.logging.HandlerTest$6</class>
//  <method>method</method>
//  <thread>1</thread>
//  <message>TEST LOG!</message>
//</record>

Level getLevel ()

Get the log level specifying which messages will be logged by this Handler.

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);
logger.setLevel(Level.ALL);

final Handler handler = new ConsoleHandler();
logger.addHandler(handler);

System.out.println(handler.getLevel()); // INFO

logger.warning("WARNING LOG!");
logger.info("INFO LOG!");
logger.fine("FINE LOG!");
logger.finer("FINER LOG!");

// Result
// ↓
//Mar 13, 2022 5:02:45 AM com.example.logging.HandlerTest$7 method
//WARNING: WARNING LOG!
//Mar 13, 2022 5:02:45 AM com.example.logging.HandlerTest$7 method
//INFO: INFO LOG!

handler.setLevel(Level.FINE);
System.out.println(handler.getLevel()); // FINE

logger.warning("WARNING LOG!");
logger.info("INFO LOG!");
logger.fine("FINE LOG!");
logger.finer("FINER LOG!");

// Result
// ↓
//Mar 13, 2022 5:02:45 AM com.example.logging.HandlerTest$7 method
//WARNING: WARNING LOG!
//Mar 13, 2022 5:02:45 AM com.example.logging.HandlerTest$7 method
//INFO: INFO LOG!
//Mar 13, 2022 5:02:45 AM com.example.logging.HandlerTest$7 method
//FINE: FINE LOG!

boolean isLoggable (LogRecord record)

Check if this Handler would actually log a given LogRecord.

final Handler handler = new ConsoleHandler();
System.out.println(handler.getLevel()); // INFO

final var ret1 = handler.isLoggable(new LogRecord(Level.WARNING, "WARNING!"));
System.out.println(ret1); // true

final var ret2 = handler.isLoggable(new LogRecord(Level.INFO, "INFO!"));
System.out.println(ret2); // true

final var ret3 = handler.isLoggable(new LogRecord(Level.FINE, "FINE!"));
System.out.println(ret3); // false

final var ret4 = handler.isLoggable(new LogRecord(Level.FINER, "FINER!"));
System.out.println(ret4); // false

abstract void publish (LogRecord record)

Publish a LogRecord.

Please see getErrorManager().

protected void reportError (String msg, Exception ex, int code)

Protected convenience method to report an error to this Handler's ErrorManager.

Please see getErrorManager().

void setEncoding (String encoding)

Set the character encoding used by this Handler.

Please see getEncoding().

void setErrorManager (ErrorManager em)

Define an ErrorManager for this Handler.

Please see getErrorManager().

void setFilter (Filter newFilter)

Set a Filter to control output on this Handler.

Please see getFilter().

void setFormatter (Formatter newFormatter)

Set a Formatter.

Please see getFormatter().

void setLevel (Level newLevel)

Set the log level specifying which message levels will be logged by this Handler.

Please see getLevel().

API Examples
- Formatter
  - SimpleFormatter
  - XMLFormatter
- Handler
- Level
- LogManager
- Logger

Java : Handler (logging) with Examples

Summary

Constructors

Handler ()

Methods

abstract void close ()

abstract void flush ()

String getEncoding ()

ErrorManager getErrorManager ()

Filter getFilter ()

Formatter getFormatter ()

Level getLevel ()

boolean isLoggable (LogRecord record)

abstract void publish (LogRecord record)

protected void reportError (String msg, Exception ex, int code)

void setEncoding (String encoding)

void setErrorManager (ErrorManager em)

void setFilter (Filter newFilter)

void setFormatter (Formatter newFormatter)

void setLevel (Level newLevel)

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