使用 try-with-resources 时是否需要调用 flush()

Question

Will try-with-resources call flush() implicitly?

If it does, in the following code snippet, bw.flush() can be safely removed?

static void printToFile1(String text, File file) {
    try (BufferedWriter bw = new BufferedWriter(new FileWriter(file))) {
        bw.write(text);
        bw.flush();
    } catch (IOException ex) {
        // handle ex
    }
}

ps. I don't see any description about it in official document:

https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html https://docs.oracle.com/javase/8/docs/api/java/lang/AutoCloseable.html

Answer 1

Closeable and AutoCloseable are general-purpose interfaces that do not know anything about flushing. So you can't find any information about it in their documentation - except some words about releasing resources .

A Writer on the other hand is a more specific-purpose abstract class that now knows something about flushing. Some excerpt of the documentation for the method Writer.close() :

Closes the stream, flushing it first .

So - yes - when using a writer, a close will always also flush . This basically means that you have to consult the documentation of the concrete classes that you are using when trying to find out what closing really does.

Answer 2

The resources are automatically closed when using try-with-resource block. As part of this process it will also invoke flush automatically.

As mentioned in doc for close method of BufferedWriter:

Closes the stream, flushing it first. Once the stream has been closed, further write() or flush() invocations will cause an IOException to be thrown.

Answer 3

Quoting javadoc of BufferedWriter.close() :

Closes the stream, flushing it first .

Answer 4

The minimum amount of code to be written in this case:

try (BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out))) {
    bw.write("Test");
} catch (IOException e) {
    // handle exception
}

Hence you don't need to call explicitly the flush method, as it will be called by the close method, as explained in the javadoc:

Closes the stream, flushing it first. Once the stream has been closed, further write() or flush() invocations will cause an IOException to be thrown. Closing a previously closed stream has no effect.

This behavior is inherited from the Writer class, hence besides BufferedWriter the same behavior is provided also by: CharArrayWriter , FilterWriter , OutputStreamWriter , PipedWriter , PrintWriter , StringWriter .

This behavior is not provided in the documentation of the tryWithResources or AutoCloseable as the behavior is specific to the given implementation of Writer . As Writer extends Closeable , it will call the close method when exiting the try {} block and the close method will first call flush as already mentioned.

Answer 5

I really don't understand why other answers focus on the BufferedWriter not try-with-resources .

I, either, couldn't find any specification or mentions that the try-with-resources statements calls flush() on any objects of Flushable .

https://docs.oracle.com/javase/specs/jls/se13/html/jls-14.html#jls-14.20.3

Don't rely on any undocumented/unspecified behavior of vendor specific implementations.

try (OutputStream o = open()) {
    //writeSome
    o.flush(); // won't hurt!
}

Answer 6

From the Javdocs:

The try-with-resources statement is a try statement that declares one or more resources. A resource is an object that must be closed after the program is finished with it. The try-with-resources statement ensures that each resource is closed at the end of the statement. Any object that implements java.lang.AutoCloseable , which includes all objects which implement java.io.Closeable , can be used as a resource.

The BufferedWriter.close() explicitly stated that:

Closes the stream, flushing it first. Once the stream has been closed, further write() or flush() invocations will cause an IOException to be thrown. Closing a previously closed stream has no effect.