java InputStream 可以连续从方法中读取数据吗？

Question

I have a piece of code

...
InputStream inputStream = new BufferedInputStream(new ByteArrayInputStream("test".getBytes()));
...

and this line makes string "test" an input for an InputStream, however this is a static InputStream. is there any way without a Scanner, System.in or user external input to make this InputStream dynamic

what I need is something like this

...
InputStream inputStream = new BufferedInputStream(new 
ByteArrayInputStream(generateContinuousDynamicString().getBytes()));
// So, basically input stream will be blocked until generateContinuousDynamicString()
// returns a result?
...

I've tried something like this

private static byte[] generateContinuousDynamicString(String s) {
    String t = "";
    // here comes the realization
    // that the source for an input stream 
    // cannot be generated dynamically on the 
    // fly it only can be read from already 
    // existing (fully generated and available 
    // resource). Am I right? Otherwise how 
    // can I adjust this method in such a way that
    // input stream would continuously have a new
    // string to read from? 
    for (int i = 0; i < 1000; i++){
        t += "<str>"+s+i+"</str>";
    }
    return ("<test>"+t+"</test>").getBytes();
}

So, if we have

...
InputStream inputStream = new BufferedInputStream(readFromADatabaseStream());
...

This is also not dynamic input stream as a resource is already in a database.

Answer 1

You want a pipe. Specifically, you want one of the following pairs of classes:

• PipedInputStream and PipedOutputStream
• PipedReader and PipedWriter

Your question asks for an InputStream, but since you're dealing with text, you probably should use a Reader, which is intended for characters. In particular, note that getBytes() will return different values on Windows systems compared to non-Windows systems, for any String with non-ASCII characters. Using a Reader and Writer will remove the need to worry about that.

Either way, the approach is the same: create the readable end of the pipe, then create and feed the writable end of the pipe in another thread.

Using a PipedReader and PipedWriter:

PipedReader pipedReader = new PipedReader();
Reader reader = new BufferedReader(pipedReader);

ExecutorService executor = Executors.newSingleThreadExecutor();
Future<?> pipeFeeder = executor.submit(
    () -> generateContinuousDynamicString(pipedReader));

// ...

private Void generateContinuousDynamicString(PipedReader pipedReader)
throws IOException {

    try (Writer writer = new PipedWriter(pipedReader)) {
        writer.write("<test>");

        for (int i = 0; i < 1000; i++) {
            writer.write("<str>" + i + "</str>");
        }

        writer.write("</test>");
    }

    return null;
}

Using a PipedInputStream and PipedOutputStream:

PipedInputStream pipedInputStream = new PipedInputStream();
InputStream inputStream = new BufferedInputStream(pipedInputStream);

ExecutorService executor = Executors.newSingleThreadExecutor();
Future<?> pipeFeeder = executor.submit(
    () -> generateContinuousDynamicString(pipedInputStream));

// ...

private Void generateContinuousDynamicString(PipedInputStream pipedInputStream)
throws IOException {

    Charset charset = StandardCharsets.UTF_8;

    try (Writer writer = new OutputStreamWriter(
            new PipedInputStream(pipedinputStream),
            StandardCharsets.UTF_8)) {

        writer.write("<test>");

        for (int i = 0; i < 1000; i++) {
            writer.write("<str>" + i + "</str>");
        }

        writer.write("</test>");
    }

    return null;
}

Answer 2

Sure. But you have a bit of an issue: Whatever code is generating the endless stream of dynamic data cannot just be in the method that 'returns the inputstream' just by itself, that's what your realisation is about.

You have two major options:

Threads

Instead, you could fire off a thread which is continually generating data. Note that whatever it 'generates' needs to be cached; this is not a good fit if, say, you want to dynamically generate an inputstream that just serves up an endless amount of 0 bytes, for example. It's a good fit if the data is coming from, say, a USB connected arduino that from time to time sends information about a temperature sensor that it's connected to. Note that you need the thread to store the data it receives someplace, and then have an inputstream that will 'pull' from this queue of data you're making. To make an inputstream that pulls from a queue, see the next section. As this will involve threads, use something from java.util.concurrent , such as ArrayBlockingQueue - this has the double benefit that you won't get infinite buffers, either (the act of putting something in the buffer will block if the buffer is full).

subclassing

What you can also do is take the code that can generate new values, but, put it in an envelope - a thing you can pass around. You want to make some code, but not run it - you want to run that later, when the thing you hand the inputstream to, calls .read() .

One easy way to do that, is to extend InputStream - and then implement your own zero method. Looks something like this:

class InfiniteZeroesInputStream extends InputStream {
    public int read() {
        return 0;
    }
}

It's that simple. Given:

try (InputStream in = new InfiniteZeroesInputStream()) {
    in.read(); // returns 0.. and will always do so.
    byte[] b = new byte[65536];
    in.read(b); // fills the whole array with zeroes.
}