Preface
In Java programming, sometimes the console's output needs to be captured as a string for subsequent processing or testing, which is especially common in logging, unit testing, or debugging. This article is mainly written because the requirement is to obtain the string printed on the console for saving. Below, we will introduce how to use it in detailByteArrayOutputStreamandPrintStreamTo implement this function.
1. Preparation phase
Before you begin,ByteArrayOutputStreamandPrintStreamUses.ByteArrayOutputStreamis an output stream that writes data into an internal byte array, which allows us to obtain these bytes as a byte array or string. andPrintStreamIs a class in the Java standard library that writes formatted output into an output stream, usually used to print information to the console.
2. Detailed operation
Next, we will conduct a point-by-point demonstration through detailed steps
2.1 Creating an output stream
First, create aByteArrayOutputStreamAn instance, used to store the output of the console. Here is a set initial capacity of 1024 bytes, but in fact this capacity is dynamically growing, so it is just a starting point.
ByteArrayOutputStream baoStream = new ByteArrayOutputStream(1024);
2.2 Create a print stream and replace system output
Next, usebaoStreamTo create aPrintStreamExample. Then,(i.e. standard output) reference is saved to a temporary variableoldStreamandSet ascacheStream, so that all information originally printed to the console will be written tobaoStreammiddle.
PrintStream cacheStream = new PrintStream(baoStream); PrintStream oldStream = ; (cacheStream); // Redirect system output to cacheStream
2.3 Execute and capture output
Through the above steps, you can then perform any operation that will be output to the console. In this example, a string is simply printed"3 1 2"。
("3 1 2");
SinceRedirected tocacheStream, so this string is actually written tobaoStream。
2.4 Recover the system output and obtain the output content
After capturing the required output, you need toRestore to its original state, i.e.oldStream. Then, you canbaoStreamGets the previously captured output content and converts it to a string.
(oldStream); // Recover the system outputString strMsg = (); // Get the output content
Note that sinceByteArrayOutputStreamoftoString()The method uses the platform's default character set by default to convert bytes to strings. If the output contains non-ASCII characters, it may be necessary to specify a character set.
2.5 Processing output content
Finally, a test is performed here to verify whether the captured content is consistent with the input, and the captured output content can be processed or asserted. In this example, try to assert whether the output content is expected to be consistent. But please note that since the printed one in the front is"3 1 2", not"2 1 3", so this assertion will fail.
// Note: The assertion here will fail because strMsg is "3 1 2" instead of "2 1 3"assertEquals("2 1 3", strMsg);
3. Summary
Through the above steps, the information that should have been printed to the console was successfully captured and converted into a string for subsequent processing. This technique is especially useful in unit testing because it allows verification that a function or method outputs the correct information as expected. At the same time, it can also be used for debugging or logging to capture the output of the program without having to rely on external tools or files. However, it should be noted that the redirection system output may have an impact on other parts of the program, so it must be restored in time after use. In general, there is generally no such demand for development, and it is not recommended to use system output during development.
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