Скачать или смотреть Piping Cat Output Directly Into A Java JAR

Learn how to effectively pipe the output of a file using `cat` into a Java JAR file and access the data seamlessly in your Java program.
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Piping Cat Output Directed Into A Java JAR

When you're working with Java applications, you might encounter scenarios where you need to supply input directly from the terminal. Specifically, if you're using a JAR file that requires data from standard input, it’s crucial to understand how to achieve this properly. One common approach is to use the cat command in a Unix-like terminal to send the contents of a file into your Java program. This guide addresses how to do this effectively while ensuring you can access the data within your application.

Understanding the Pipe

When you run a command like:

[[See Video to Reveal this Text or Code Snippet]]

You are using the pipe operator (|) to redirect the output of the cat command into your Java application. However, a common misconception is that this data is accessible via the String[] args parameter in the main method of your Java class. Unfortunately, this cannot be done. Instead, the data from the cat filename command is passed through a different channel: Standard Input (System.in).

Why You Can’t Use String[] args

The typical way to receive input in Java applications is through command-line arguments defined by String[] args. However, when piping data from one command to another, the piped data does not become part of this array. Instead, it flows through the input stream that the program can read from, which is System.in. Recognizing this distinction is essential for managing input data in your Java program.

Reading Data from System.in

To read piped data in your Java application, you will need to utilize classes designed to handle standard input. Below is a straightforward approach to doing this.

Example Java Program

The following Java program demonstrates how to read from System.in and output the standard input back to the console with line numbers:

[[See Video to Reveal this Text or Code Snippet]]

How It Works

BufferedReader and InputStreamReader: These two classes are used to create a reader that can process character streams efficiently.

Reading Lines: The program reads each line until it encounters the end of the input (when the input is null).

Line Numbering: It adds simple line numbering for easier readability.

Running the Program

To see this in action, create a text file named hello.txt with the following content:

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Run the following command to pipe the contents into your Java application:

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You should see output similar to this:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Piping the output of command-line tools into Java applications opens up powerful possibilities for automation and data processing. By correctly using System.in, you can effectively utilize the input data and make your Java applications more dynamic and responsive to external data.

By following the guidelines provided above, you should now have a clearer understanding of how to effectively pass input to a Java JAR via the terminal using cat and the concept of System.in.