java/kudu-test-utils/src/main/java/org/apache/kudu/test/CapturingLogAppender.java

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//
//   http://www.apache.org/licenses/LICENSE-2.0
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package org.apache.kudu.test;

import java.io.Closeable;
import java.io.IOException;
import java.util.Random;
import javax.annotation.concurrent.GuardedBy;

import com.google.common.base.Throwables;
import org.apache.logging.log4j.core.LogEvent;
import org.apache.logging.log4j.core.LoggerContext;
import org.apache.logging.log4j.core.appender.AbstractAppender;
import org.apache.logging.log4j.core.config.Property;
import org.apache.logging.log4j.core.layout.PatternLayout;
import org.apache.yetus.audience.InterfaceAudience;
import org.apache.yetus.audience.InterfaceStability;

/**
 * Test utility which wraps Log4j and captures all messages logged
 * while it is attached. This can be useful for asserting that a particular
 * message is (or is not) logged.
 */
@InterfaceAudience.Private
@InterfaceStability.Unstable
public class CapturingLogAppender extends AbstractAppender {
  // This is the standard layout used in Kudu tests.
  private static final PatternLayout LAYOUT = PatternLayout.newBuilder()
      .withPattern("%d{HH:mm:ss.SSS} [%p - %t] (%F:%L) %m%n")
      .build();

  // The caller should detach the logger before calling getAppendedText().
  // Nevertheless, for some reason it is still possible for additional
  // append() calls to happen _after_ the logger is detached, which may race
  // with getAppendedText().
  @GuardedBy("this")
  private StringBuilder appended = new StringBuilder();

  public CapturingLogAppender() {
    // Appender name must be unique so that attaching/detaching works correctly
    // when multiple capturing appenders are used recursively.
    super(String.format("CapturingToFileLogAppender-%d", new Random().nextInt()),
          /* filter */ null, LAYOUT, /* ignoreExceptions */ true, Property.EMPTY_ARRAY);

    // If we don't call start(), we get an ugly log error:
    //
    // ERROR Attempted to append to non-started appender CapturingToFileLogAppender
    start();
  }

  @Override
  public synchronized void append(LogEvent event) {
    appended.append(getLayout().toSerializable(event));
    if (event.getThrown() != null) {
      appended.append(Throwables.getStackTraceAsString(event.getThrown()));
      appended.append("\n");
    }
  }

  /**
   * @return all of the appended messages captured thus far, joined together.
   */
  public synchronized String getAppendedText() {
    return appended.toString();
  }

  /**
   * Temporarily attach the capturing appender to the Log4j root logger.
   * This can be used in a 'try-with-resources' block:
   * <code>
   *   try (Closeable c = capturer.attach()) {
   *     ...
   *   }
   * </code>
   */
  public Closeable attach() {
    LoggerContext.getContext(false).getRootLogger().addAppender(this);
    return new Closeable() {
      @Override
      public void close() throws IOException {
        LoggerContext.getContext(false).getRootLogger().removeAppender(CapturingLogAppender.this);
      }
    };
  }
}