Java : StreamHandler (logging) with Examples

final StreamHandler handler = new ConsoleHandler();

System.out.println(handler.getLevel()); // INFO
System.out.println(handler.getFormatter()); // java.util.logging.SimpleFormatter@6be968ce
System.out.println(handler.getFilter()); // null

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

logger.info("TEST LOG!");

// Result
// ↓
//Jan 10, 2023 11:25:49 PM com.example.logging.StreamHandlerTest$1 method
//INFO: TEST LOG!

StreamHandler ()

Create a StreamHandler, with no current output stream.

final var outputStream = new ByteArrayOutputStream();

class SampleStreamHandler extends StreamHandler {
    SampleStreamHandler() {
        super();
        setOutputStream(outputStream);
    }
}

final var handler = new SampleStreamHandler();

System.out.println(handler.getLevel()); // INFO
System.out.println(handler.getFormatter()); // java.util.logging.SimpleFormatter@76d7e04

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

logger.info("TEST LOG!");
handler.flush();

System.out.print(outputStream);

// Result
// ↓
//Mar 14, 2022 1:47:29 AM com.example.logging.StreamHandlerTest$1 method
//INFO: TEST LOG!

StreamHandler (OutputStream out, Formatter formatter)

Create a StreamHandler with a given Formatter and output stream.

final var outputStream = new ByteArrayOutputStream();

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

System.out.println(handler.getLevel()); // INFO
System.out.println(handler.getFormatter()); // java.util.logging.SimpleFormatter@84e9fc3

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

logger.info("TEST LOG!");
handler.flush();

System.out.print(outputStream);

// Result
// ↓
//Mar 14, 2022 1:50:07 AM com.example.logging.StreamHandlerTest$2 method
//INFO: TEST LOG!

void close ()

Close the current output stream.

final var pattern = Path.of("R:", "java-work", "log.txt");

final StreamHandler handler = new FileHandler(pattern.toString());
handler.setFormatter(new SimpleFormatter());

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

logger.info("BEFORE CLOSE LOG!");
System.out.println(Files.readString(pattern));

// Result
// ↓
//Mar 14, 2022 1:54:26 AM com.example.logging.StreamHandlerTest$3 method
//INFO: BEFORE CLOSE LOG!

handler.close();

logger.info("AFTER CLOSE LOG!");
System.out.println(Files.readString(pattern));

// Result (Not output "AFTER CLOSE LOG!")
// ↓
//Mar 14, 2022 1:54:26 AM com.example.logging.StreamHandlerTest$3 method
//INFO: BEFORE CLOSE LOG!

void flush ()

Flush any buffered messages.

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

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);
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 14, 2022 2:30:08 AM com.example.logging.StreamHandlerTest$4 method
//INFO: TEST LOG!

boolean isLoggable (LogRecord record)

Check if this Handler would actually log a given LogRecord.

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

final var handler = new StreamHandler(new ByteArrayOutputStream(), new SimpleFormatter());
final var logRecord = new LogRecord(Level.INFO, "TEST LOG!");

System.out.println(handler.isLoggable(logRecord)); // true

handler.close();

System.out.println(handler.isLoggable(logRecord)); // false

void publish (LogRecord record)

Format and publish a LogRecord.

final var outputStream = new ByteArrayOutputStream();
final var handler = new StreamHandler(outputStream, new SimpleFormatter()) {
    @Override
    public synchronized void publish(LogRecord logRecord) {
        logRecord.setMessage(logRecord.getMessage() + "：override publish!");

        super.publish(logRecord);
    }
};

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

logger.info("TEST LOG!");
handler.flush();

System.out.print(outputStream);

// Result
// ↓
//Mar 14, 2022 2:34:43 AM com.example.logging.StreamHandlerTest$5 method
//INFO: TEST LOG!：override publish!

void setEncoding (String encoding)

Set (or change) the character encoding used by this Handler.

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

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

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

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

logger.addHandler(handler);

logger.info("TEST LOG : 〇△□×");
handler.flush();

// An example of outputting to the console for UTF-8.
System.out.print(outputStream);

// Result
// ↓
//Mar 14, 2022 2:35:20 AM com.example.logging.StreamHandlerTest$6 method
//INFO: TEST LOG : 〇△□×

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

final var outputStream = new ByteArrayOutputStream();
final var handler = new StreamHandler(outputStream, new SimpleFormatter());
logger.addHandler(handler);

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

logger.info("TEST LOG : 〇△□×");
handler.flush();

// An example of outputting to the console for UTF-8.
System.out.print(outputStream);

// Result (Garbled characters)
// ↓
//Mar 14, 2022 2:38:29 AM com.example.logging.StreamHandlerTest$7 method
//INFO: TEST LOG : �Z�����~

protected void setOutputStream (OutputStream out)

Change the output stream.

Please see StreamHandler().

getEncoding, getErrorManager, getFilter, getFormatter, getLevel, reportError, setErrorManager, setFilter, setFormatter, setLevel

Please see the link below.

• Java API Examples : Handler