What is the purpose of flush() in Java streams?

java

buffering

io-performance

best-practices

byAlex Kataev·Mar 12, 2025

In the realm of Java, the flush() method plays a critical role when it comes to high-precision I/O operations. This fantastic method ensures all buffered data hiding within any output stream are immediately and safely sent out to their intended destination. This could be a file, a network socket, or whatever your program is talking to.

For instance:

BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt"));
writer.write("Flush ensures this gets written...like, right away!");
writer.flush();  // Now you see me!
writer.close();  // Now you don't!

Key Concepts:

flush() for immediate data release from buffer.
Absolute safety from data loss.
Particularly handy with output streams like BufferedWriter.

Importance of buffering

Buffering is the knight in shining armor that saves your data from getting lost in the wilds of I/O operations, providing a safe and temporary space for data to gather before being written out in a single, larger operation. This clever technique boosts I/O performance by cutting down the number of write operations. However, prompt delivery of data is ensured by our hero - flush()!

The when and where of flush()

Especially useful when you are going for real-time applications such as chatting apps, where every single data point can bring a smile, or sophisticated data processing systems, where every nanosecond is money. Failing to flush can correspond to lost moments or missed opportunities!

Buffer judo tricks

Mastering buffer sizes is a vital aspect of efficient I/O operations. By default, classes like BufferedWriter get a buffer of size 8192 bytes, which works pretty well under ordinary circumstances. But for those who seek the extraordinary, understanding buffer sizes can be enlightening.

An expansive buffer optimizes throughput for large data, saving time by minimizing disk I/O operations.
A minimal buffer works wonders for low latency applications or scenarios involving less data.

Scenarios screaming for a flush

Before you send goodbye signals: Since not every close() operation flushes the buffered data, a flush() before a close() ensures every story is heard.
Swing-ing with UI: Keeping GUI elements updated that are interconnected with underlying stream objects.
Keeping tabs: Correctly reflecting status updates and errors when dealing with log files, making debugging a breeze.

Debugging and tuning tips with flush

Buffer judo tricks not enough? Here's some more pro level advice:

Experiment with Buffer Sizes: You can tweak buffer sizes and see how your application's performance responds.
Keep an Eye on Resources: Make sure your I/O operations aren't choking the life out of your system resources.
Make Life Easier with Autocloseable Resources: With a try-with-resources statement, BufferedWriter can automatically flush() and close() with less line of code.
Flush Before Concurrency: To maintain data consistency, calling flush() before using concurrency control constructs like synchronized, Locks, or Semaphores is a good practice.