When to use which Writer subclass in Java; common practices

问题

I have always been slightly confused with the amount of different IO implementations in Java, and now that I am completely stuck in my project development, I was taking my time to read up on useful stuff meanwhile.

I have realized that there is no newbie-friendly comparison (apart from short explanation at API for Writer class) between the different subclasses of the Writer class. So I figured I'd fire away the question, what are those different subclasses good for?

For example, I usually use a FileWriter wrapped with a BufferedWriter for my outputs to files but I have always been irritated by the fact that there is no println() like method, and one has to use newLine() every second line (to make the output human readable). PrintWriterhas the println() method but no constructor that supports appending however...

I'd really appreciate if you could give me your two cents from your experience, or a nice guide/how-to you might have stumbled upon.

EDIT: Thanks for the replies everyone, I really appreciate the info passed on here. It's a bit unfortunate that the whole append() thing ended up being in focus, it merely meant it as an example. My question was mostly referring to the need and use of all the different implementations, which I guess was mentioned somewhat in a couple of the answers.

It's hard to pick one answer as accepted, since there are three really solid answers, each has contributed to my understanding of the problem. I am gonna have to go with Anon, this time as he's got the least amount of rep. points (I presume he's new on SO). He's 15 answers some of which are really well formulated, and 0 questions asked. Good contribution I'd say, and that is worth promoting.

That being said, ColinD and Jay also provided really good answers, and have pointed out interesting ideas. Especially Jay's comment about Java automatically wrapping a BufferedWriter was worth noting. Thanks again guys, really appreciated!

回答1:

The java.io classes generally follow the Decorator pattern. So, while PrintWriter does not have the specific constructor you might want, it does have a constructor that takes another Writer, so you can do something like the following:

FileOutputStream fos = null;
try
{
    fos = new FileOutputStream("foo.txt");
    PrintWriter out = new PrintWriter(
                          new BufferedWriter(
                              new OutputStreamWriter(fos, "UTF-8")));
    // do what you want to do
    out.flush();
    out.close();
}
finally
{
    // quietly close the FileOutputStream (see Jakarta Commons IOUtils)
}

As a general usage note, you always want to wrap a low-level Writer (eg FileWriter or OutputStreamWriter) in a BufferedWriter, to minimize actual IO operations. However, this means that you need to explicitly flush and close the outermost Writer, to ensure that all content is written.

And then you need to close the low-level Writer in a finally block, to ensure that you don't leak resources.

Edit:

Looking at MForster's answer made me take another look at the API for FileWriter. And I realized that it doesn't take an explicit character set, which is a Very Bad Thing. So I've edited my code snippet to use a FileOutputStream wrapped by an OutputStreamWriter that takes an explicit character set.

回答2:

FileWriter is generally not an acceptable class to use. It does not allow you to specify the Charset to use for writing, which means you are stuck with whatever the default charset of the platform you're running on happens to be. Needless to say, this makes it impossible to consistently use the same charset for reading and writing text files and can lead to corrupted data.

Rather than using FileWriter, you should be wrapping a FileOutputStream in an OutputStreamWriter. OutputStreamWriter does allow you to specify a charset:

File file = ...
OutputStream fileOut = new FileOutputStream(file);
Writer writer = new BufferedWriter(new OutputStreamWriter(fileOut, "UTF-8"));

To use PrintWriter with the above, just wrap the BufferedWriter in a PrintWriter:

PrintWriter printWriter = new PrintWriter(writer);

You could also just use the PrintWriter constructor that takes a File and the name of a charset:

PrintWriter printWriter = new PrintWriter(file, "UTF-8");

This works just fine for your particular situation, and actually does the exact same thing as the code above, but it's good to know how to build it by wrapping the various parts.

The other Writer types are mostly for specialized uses:

• StringWriter is just a Writer that can be used to create a String. CharArrayWriter is the same for char[].
• PipedWriter for piping to a PipedReader.

Edit:

I noticed that you commented on another answer about the verbosity of creating a writer this way. Note that there are libraries such as Guava that help reduce the verbosity of common operations. Take, for example, writing a String to a file in a specific charset. With Guava you can just write:

Files.write(text, file, Charsets.UTF_8);

You can also create a BufferedWriter like this:

BufferedWriter writer = Files.newWriter(file, Charsets.UTF_8);

回答3:

PrintWriter doesn't have a constructor that takes an "append" parameter, but FileWriter does. And it seems logical to me that that's where it belongs. PrintWriter doesn't know if you're writing to a file, a socket, the console, a string, etc. What would it mean to "append" on writes to a socket?

So the right way to do what you want is simply:

PrintWriter out=new PrintWriter(new BufferedWriter(new FileWriter(myfile, append)));

Interesting side note: If you wrap an OutputStream in a PrintWriter, Java automatically inserts a BufferedWriter in the middle. But if you wrap a Writer in a PrintWriter, it does not. So nothing is gained by saying:

PrintWriter out=new PrintWriter(new BufferedWriter(new OutputStreamWriter(new FileOutputStream(myfile))));

Just leave off the BufferedWriter and the OutputStreamWriter, you get them for free anyway. I have no idea if there is some good reason for the inconsistency.

It's true that you can't specify a character encoding in a FileWriter as ColinD notes. I don't know that that makes it "unacceptable". I almost always am perfectly happy to accept the default encoding. Maybe if you're using a language other than English this is an issue.

The need to wrap Writers or OutputStreams in layers was confusing to me when I first started using Java. But once you get the hang of it, it's no big deal. You just have to bend your mind into the write framework. Each writer has a function. Think of it like, I want to print to a file, so I need to wrap a FileWriter in a PrintWriter. Or, I want to convert an output stream to a writer, so I need an OutputStreamWriter. Etc.

Or maybe you just get used to the ones you use all the time. Figure it out once and remember how you did it.

回答4:

You can create an appending PrintWriter like this:

OutputStream os = new FileOutputStream("/tmp/out", true);
PrintWriter writer = new PrintWriter(os);

Edit: Anon's post is right about both using a BufferedWriter in between and specifying the encoding.

来源：https://stackoverflow.com/questions/4069698/when-to-use-which-writer-subclass-in-java-common-practices