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The Exchanger and GC-less Java

Overview

The Exchanger class is very efficient at passing work between thread and recycling the objects used. AFAIK, It is also one of the least used Concurrency classes.

Nevertheless, if you don’t need GC less logging using an ArrayBlockingQueue is much simpler.

Exchanger class

The Exchanger class is useful for passing data back and forth between two threads. e.g. Producer/Consumer. It has the property of naturally recycling the data structures used to pass the work and supports GC-less sharing of work in an efficient manner.

Here is an example, passing logs to a background logger.

Work (a log entry) is batched into LogEntries and passed to a background thread which later passes it back to the thread so it can add more work. Provided the background thread is always finished before the batch is full, it is almost transparent. Increasing the size of the batch reduces how often the batch is full but increase the number of unprocessed entries waiting at any one time. Calling flush() can push out the data.

The key line is the following which exchanges the batch in the current thread with the batch in the other thread. The producer fills up the batch while the consumer is emptying it.

The exchange when it occurs typically takes 1-4 micro-seconds. In this case, once every 64 lines.

entries = logEntriesExchanger.exchange(entries);

Exchanger example

import java.util.concurrent.Exchanger;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class BackgroundLogger implements Runnable {
  static final int ENTRIES = 64;

  static class LogEntry {
    long time;
    int level;
    final StringBuilder text = new StringBuilder();
  }

  static class LogEntries {
    final LogEntry[] lines = new LogEntry[ENTRIES];
    int used = 0;
  }

  private final ExecutorService executor = Executors.newSingleThreadExecutor();
  final Exchanger<LogEntries> logEntriesExchanger = new Exchanger<LogEntries>();
  LogEntries entries = new LogEntries();

  BackgroundLogger() {
    executor.submit(this);
  }

  public StringBuilder log(int level) {
    try {
      if (entries.used == ENTRIES)
        entries = logEntriesExchanger.exchange(entries);
      LogEntry le = entries.lines[entries.used++];
      le.time = System.currentTimeMillis();
      le.level = level;
      return le.text;

    } catch (InterruptedException e) {
      throw new RuntimeException(e);
    }
  }

  public void flush() throws InterruptedException {
    if(entries.used > 0)
        entries = logEntriesExchanger.exchange(entries);
  }

  public void stop() {
    try {
      flush();
    } catch (InterruptedException e) {
      e.printStackTrace(); // use standard logging.
    }
    executor.shutdownNow();
  }

  @Override
  public void run() {
    LogEntries entries = new LogEntries();
    try {
      while (!Thread.interrupted()) {
        entries = logEntriesExchanger.exchange(entries);
            for (int i = 0; i < entries.used; i++) {
              bgLog(entries.lines[i]);
              entries.lines[i].text.delete(0, entries.lines[i].text.length());
        }
        entries.used = 0;
      }
    } catch (InterruptedException ignored) {

    } finally {
      System.out.println("Warn: logger stopping."); // use standard logging.
    }
  }

  private void bgLog(LogEntry line) {
    // log the entry to a file.
  }
}

Reference: The Exchanger and GC-less Java from our JCG partner Peter Lawrey at the Vanilla Java.

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2 comments

  1. Is this thread safe?
    It looks to me that lines 29 to 31 are not. Am I wrong?

  2. The entire method is not thread safe with respect to other callers of that method.

    It is only thread safe with respect to the thread on the other end of the exchanger.

    i.e. provide only one thread is attempting to use log() AND the StringBuilder it returns, it will behave correctly.

    BTW There is much more efficient libraries to do this such as Java Chronicle which supports concurrent writers and reader to log file across processes.

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