InputStream.available() strange behavior

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01-09-2024

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Understanding the InputStream.available() Quirks: A Practical Guide

The InputStream.available() method in Java is often used to determine the number of bytes available for reading from an input stream. However, its behavior can be unpredictable, leading to unexpected results. This article explores the nuances of InputStream.available() and offers practical solutions to avoid common pitfalls, especially when dealing with existing Java code you cannot modify.

The Challenge: InputStream.available() Returning 0

Imagine this scenario: You're developing a backend system to interact with an existing Java application. This application relies on InputStream.available() to determine if data is available for processing. However, the method consistently returns 0, even though data is actually present. This situation presents a major hurdle, as you cannot change the Java code.

This scenario is exactly what was faced by a developer on Stack Overflow https://stackoverflow.com/questions/4876329/inputstream-available-doesnt-work. The developer was using InputStream.available() to check for data downloaded from a remote server. The method consistently returned 0, despite the data being successfully downloaded.

Why InputStream.available() Can Be Unreliable

The crux of the issue lies in the way InputStream.available() functions:

• It's a best-effort estimate: The method provides an indication of the potential number of bytes available. It may not accurately reflect the true amount of data ready to be read.
• It's not a reliable indicator for all streams: InputStream.available() is often unreliable with network streams, such as those used for downloading data from a server. These streams may be buffered, meaning data arrives in chunks.
• It can be influenced by buffering: The buffering strategy of the underlying input stream significantly impacts InputStream.available().

Workarounds: Getting Data Despite InputStream.available()'s Limitations

Given the limitations of InputStream.available(), how can you ensure your backend system correctly handles data received from the Java application?

Here's a practical approach based on the Stack Overflow example:

1. Embrace the readNBytes() Method: As demonstrated in the Stack Overflow example, using InputStream.readNBytes() is a reliable way to read data, even when InputStream.available() returns 0. You can read a predetermined number of bytes (e.g., 2048 * 1024) and process the data.

2. Implement a Loop with a Timeout: This approach involves reading data in chunks until a timeout is reached or all data is read. The code snippet below demonstrates this:

   int timeout = 1000; // Timeout in milliseconds
   long startTime = System.currentTimeMillis();
   StringBuilder sb = new StringBuilder();
   byte[] buffer = new byte[1024];
   int bytesRead;
   while ((bytesRead = is.read(buffer)) != -1 && System.currentTimeMillis() - startTime < timeout) {
       sb.append(new String(buffer, 0, bytesRead));
   }
   String content = sb.toString();
   // Process the content
   

   This code reads data until either all data is read or the timeout is reached. This approach works well for scenarios where the data size is unknown or potentially variable.

Key Takeaways

• InputStream.available() is not always reliable, especially with network streams.
• Rely on methods like readNBytes() or loop-based approaches to read data accurately.
• Understand that these workarounds might be less elegant, but they offer practical solutions when modifying existing Java code is impossible.

This article has shed light on the unpredictable nature of InputStream.available() and explored practical ways to handle data from Java applications even when this method returns unexpected results. By understanding these complexities and employing the right strategies, you can build robust backend systems that reliably interact with existing Java code, ensuring smooth and consistent data processing.