How are Java I/O streams used in a multi-threaded environment?

Thread safety needs to be considered when using Java I/O streams in a multi-threaded environment. There are three methods to ensure safety: 1. Synchronous I/O operations; 2. Use thread local variables to provide independent I for each thread. /O object; 3. For situations where multiple I/O operations need to be processed, a concurrent queue can be used. One thread puts the operation into the queue, and another thread takes it out of the queue and executes it.

The use of Java I/O streams in multi-threaded environments

Preface

Using Java I/O streams in a multi-threaded environment requires special attention to thread safety issues. If used incorrectly, data corruption or inconsistency may result. This article will delve into the correct usage of I/O streams in multi-threading and demonstrate it through practical cases.

Synchronization

The simplest way to ensure thread safety is Synchronization I/O operations. For read and write operations involving the same file, use the following code block to synchronize them:

synchronized (file) {
  // 读写操作
}

Thread-local variables

Another approach is to use Thread-local Variable (ThreadLocal), provides each thread with a separate copy of the I/O object. In this way, each thread can safely access its own I/O objects without conflicting with other threads:

public class IOUtil {

  private static final ThreadLocal<BufferedInputStream> inputStream = new ThreadLocal<>();

  public static BufferedInputStream getInputStream(String fileName) {
    synchronized (inputStream) {
      BufferedInputStream stream = inputStream.get();
      if (stream == null) {
        stream = new BufferedInputStream(new FileInputStream(fileName));
        inputStream.set(stream);
      }
      return stream;
    }
  }

}

Concurrent Queue

For those who need to process In the case of multiple I/O operations, you can use Concurrent Queue (BlockingQueue). Each thread can put I/O operations into the queue, and another thread can take them off the queue and perform these operations:

public class FileProcessor implements Runnable {

  private BlockingQueue<String> fileQueue;

  @Override
  public void run() {
    while (!fileQueue.isEmpty()) {
      String fileName = fileQueue.take();
      try (BufferedReader in = new BufferedReader(new FileReader(fileName))) {
        // 处理文件
      } catch (IOException e) {
        // 处理异常
      }
    }
  }

}

Practical Case

Scenario: Multiple threads read the contents of the same file at the same time and output it to the console.

Implementation:

import java.io.*;
import java.util.concurrent.*;

public class MultiThreadRead {

  private static final String FILE_NAME = "test.txt";
  private static final int NUM_THREADS = 4;

  public static void main(String[] args) {
    ExecutorService executor = Executors.newFixedThreadPool(NUM_THREADS);
    BlockingQueue<String> fileQueue = new ArrayBlockingQueue<>(NUM_THREADS);

    try (BufferedReader in = new BufferedReader(new FileReader(FILE_NAME))) {
      String line;
      while ((line = in.readLine()) != null) {
        fileQueue.put(line);
      }
    } catch (IOException e) {
      e.printStackTrace();
    }

    for (int i = 0; i < NUM_THREADS; i++) {
      executor.execute(new FileProcessor(fileQueue));
    }

    executor.shutdown();
  }

}

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