是否可以从具有超时的InputStream读取?

时间:2009-04-30 00:52:53

标签: java timeout inputstream

具体来说,问题是写一个这样的方法:

int maybeRead(InputStream in, long timeout)

如果数据在'timeout'毫秒内可用,则返回值与in.read()相同,否则为-2。在方法返回之前,任何生成的线程都必须退出。

为了避免参数,这里的主题是java.io.InputStream,由Sun(任何Java版本)记录。请注意,这并不像看起来那么简单。以下是Sun的文档直接支持的一些事实。

  1. in.read()方法可能是不可中断的。

  2. 在Reader或InterruptibleChannel中包装InputStream没有帮助,因为所有这些类都可以调用InputStream的方法。如果可以使用这些类,则可以编写一个直接在InputStream上执行相同逻辑的解决方案。

  3. in.available()始终可以返回0。

  4. in.close()方法可能会阻止或不执行任何操作。

  5. 没有通用的方法来杀死另一个线程。

8 个答案:

答案 0 :(得分:75)

使用inputStream.available()

  

System.in.available()始终可以返回0。

我发现了相反的情况 - 它总是返回可用字节数的最佳值。 InputStream.available()的Javadoc:

Returns an estimate of the number of bytes that can be read (or skipped over) 
from this input stream without blocking by the next invocation of a method for 
this input stream.

由于时间/陈旧性,估计是不可避免的。这个数字可能是一次性的低估,因为新的数据不断到来。然而,它总是在接下来的呼叫中“赶上” - 它应该考虑所有到达的数据,即在新呼叫时刻到达的数据栏。当有数据失败时,永久性地返回0。

第一个警告:InputStream的具体子类负责可用()

InputStream是一个抽象类。它没有数据源。拥有可用数据对它毫无意义。因此,available()的javadoc也声明:

The available method for class InputStream always returns 0.

This method should be overridden by subclasses.

实际上,具体的输入流类会覆盖available(),提供有意义的值,而不是常量0。

第二个警告:确保在Windows中输入输入时使用回车符。

如果使用System.in,程序只会在命令shell交出时接收输入。如果您正在使用文件重定向/管道(例如somefile&gt; java myJavaApp或somecommand | java myJavaApp),则输入数据通常会立即移交。但是,如果您手动键入输入,则可能会延迟数据切换。例如。使用Windows cmd.exe shell,数据在cmd.exe shell中缓冲。数据仅在回车后传递给执行的java程序(control-m或<enter>)。这是执行环境的限制。当然,只要shell缓冲数据,InputStream.available()就会返回0 - 这是正确的行为;那时没有可用的数据。一旦数据可从shell获得,该方法返回值&gt;注意:Cygwin也使用cmd.exe。

最简单的解决方案(无阻塞,因此无需超时)

请使用:

    byte[] inputData = new byte[1024];
    int result = is.read(inputData, 0, is.available());  
    // result will indicate number of bytes read; -1 for EOF with no data read.

OR等效,

    BufferedReader br = new BufferedReader(new InputStreamReader(System.in, Charset.forName("ISO-8859-1")),1024);
    // ...
         // inside some iteration / processing logic:
         if (br.ready()) {
             int readCount = br.read(inputData, bufferOffset, inputData.length-bufferOffset);
         }

更丰富的解决方案(在超时期限内最大限度地填充缓冲区)

声明:

public static int readInputStreamWithTimeout(InputStream is, byte[] b, int timeoutMillis)
     throws IOException  {
     int bufferOffset = 0;
     long maxTimeMillis = System.currentTimeMillis() + timeoutMillis;
     while (System.currentTimeMillis() < maxTimeMillis && bufferOffset < b.length) {
         int readLength = java.lang.Math.min(is.available(),b.length-bufferOffset);
         // can alternatively use bufferedReader, guarded by isReady():
         int readResult = is.read(b, bufferOffset, readLength);
         if (readResult == -1) break;
         bufferOffset += readResult;
     }
     return bufferOffset;
 }

然后使用:

    byte[] inputData = new byte[1024];
    int readCount = readInputStreamWithTimeout(System.in, inputData, 6000);  // 6 second timeout
    // readCount will indicate number of bytes read; -1 for EOF with no data read.

答案 1 :(得分:64)

假设您的流没有套接字支持(因此您无法使用Socket.setSoTimeout()),我认为解决此类问题的标准方法是使用Future。

假设我有以下执行程序和流:

    ExecutorService executor = Executors.newFixedThreadPool(2);
    final PipedOutputStream outputStream = new PipedOutputStream();
    final PipedInputStream inputStream = new PipedInputStream(outputStream);

我有编写器写入一些数据,然后在写入最后一段数据并关闭流之前等待5秒:

    Runnable writeTask = new Runnable() {
        @Override
        public void run() {
            try {
                outputStream.write(1);
                outputStream.write(2);
                Thread.sleep(5000);
                outputStream.write(3);
                outputStream.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    };
    executor.submit(writeTask);

正常的阅读方式如下。读取将无限期地阻塞数据,因此在5s内完成:

    long start = currentTimeMillis();
    int readByte = 1;
    // Read data without timeout
    while (readByte >= 0) {
        readByte = inputStream.read();
        if (readByte >= 0)
            System.out.println("Read: " + readByte);
    }
    System.out.println("Complete in " + (currentTimeMillis() - start) + "ms");

输出:

Read: 1
Read: 2
Read: 3
Complete in 5001ms

如果有一个更基本的问题,比如作者没有回应,读者就会永远阻止。 如果我将来包装读取,我可以按如下方式控制超时:

    int readByte = 1;
    // Read data with timeout
    Callable<Integer> readTask = new Callable<Integer>() {
        @Override
        public Integer call() throws Exception {
            return inputStream.read();
        }
    };
    while (readByte >= 0) {
        Future<Integer> future = executor.submit(readTask);
        readByte = future.get(1000, TimeUnit.MILLISECONDS);
        if (readByte >= 0)
            System.out.println("Read: " + readByte);
    }

输出:

Read: 1
Read: 2
Exception in thread "main" java.util.concurrent.TimeoutException
    at java.util.concurrent.FutureTask$Sync.innerGet(FutureTask.java:228)
    at java.util.concurrent.FutureTask.get(FutureTask.java:91)
    at test.InputStreamWithTimeoutTest.main(InputStreamWithTimeoutTest.java:74)

我可以捕获TimeoutException并进行任何我想要的清理。

答案 2 :(得分:18)

如果您的InputStream由Socket支持,您可以使用setSoTimeout设置套接字超时(以毫秒为单位)。如果read()调用在指定的超时内没有解除阻塞,它将抛出一个SocketTimeoutException。

在进行read()调用之前,请确保在Socket上调用setSoTimeout。

答案 3 :(得分:18)

我会质疑问题陈述而不是盲目地接受它。您只需要从控制台或通过网络超时。如果后者你有Socket.setSoTimeout()HttpURLConnection.setReadTimeout()两者都完全符合要求,只要你在构造/获取它们时正确设置它们。当你拥有所有东西时,将它留在应用程序的后面的任意点是糟糕的设计导致非常笨拙的实现。

答案 4 :(得分:7)

我没有使用过Java NIO包中的类,但似乎它们可能在这里有所帮助。具体而言,java.nio.channels.Channelsjava.nio.channels.InterruptibleChannel

答案 5 :(得分:5)

这是一种从System.in获取NIO FileChannel并使用超时检查数据可用性的方法,这是问题中描述的问题的特例。在控制台运行它,不要输入任何输入,然后等待结果。它在Windows和Linux上的Java 6下成功测试过。

import java.io.FileInputStream;
import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.Field;
import java.nio.ByteBuffer;
import java.nio.channels.ClosedByInterruptException;

public class Main {

    static final ByteBuffer buf = ByteBuffer.allocate(4096);

    public static void main(String[] args) {

        long timeout = 1000 * 5;

        try {
            InputStream in = extract(System.in);
            if (! (in instanceof FileInputStream))
                throw new RuntimeException(
                        "Could not extract a FileInputStream from STDIN.");

            try {
                int ret = maybeAvailable((FileInputStream)in, timeout);
                System.out.println(
                        Integer.toString(ret) + " bytes were read.");

            } finally {
                in.close();
            }

        } catch (Exception e) {
            throw new RuntimeException(e);
        }

    }

    /* unravels all layers of FilterInputStream wrappers to get to the
     * core InputStream
     */
    public static InputStream extract(InputStream in)
            throws NoSuchFieldException, IllegalAccessException {

        Field f = FilterInputStream.class.getDeclaredField("in");
        f.setAccessible(true);

        while( in instanceof FilterInputStream )
            in = (InputStream)f.get((FilterInputStream)in);

        return in;
    }

    /* Returns the number of bytes which could be read from the stream,
     * timing out after the specified number of milliseconds.
     * Returns 0 on timeout (because no bytes could be read)
     * and -1 for end of stream.
     */
    public static int maybeAvailable(final FileInputStream in, long timeout)
            throws IOException, InterruptedException {

        final int[] dataReady = {0};
        final IOException[] maybeException = {null};
        final Thread reader = new Thread() {
            public void run() {                
                try {
                    dataReady[0] = in.getChannel().read(buf);
                } catch (ClosedByInterruptException e) {
                    System.err.println("Reader interrupted.");
                } catch (IOException e) {
                    maybeException[0] = e;
                }
            }
        };

        Thread interruptor = new Thread() {
            public void run() {
                reader.interrupt();
            }
        };

        reader.start();
        for(;;) {

            reader.join(timeout);
            if (!reader.isAlive())
                break;

            interruptor.start();
            interruptor.join(1000);
            reader.join(1000);
            if (!reader.isAlive())
                break;

            System.err.println("We're hung");
            System.exit(1);
        }

        if ( maybeException[0] != null )
            throw maybeException[0];

        return dataReady[0];
    }
}

有趣的是,当在NetBeans 6.5中而不是在控制台上运行程序时,超时根本不起作用,并且实际上需要调用System.exit()来杀死僵尸线程。会发生什么事情是中断线程在调用reader.interrupt()时阻塞(!)。另一个测试程序(此处未显示)另外尝试关闭通道,但这也不起作用。

答案 6 :(得分:4)

正如jt所说,NIO是最好的(也是正确的)解决方案。如果你真的被一个InputStream困住了,你可以

  1. 产生一个独占作业的线程是从InputStream中读取并将结果放入一个缓冲区,该缓冲区可以从原始线程中读取而不会阻塞。如果您只有一个流实例,这应该可以正常工作。否则,您可以使用Thread类中的弃用方法终止线程,但这可能会导致资源泄漏。

  2. 依赖isAvailable指示可以不受阻塞地读取的数据。但是在某些情况下(例如使用套接字),可能需要对isAvailable进行阻塞读取,以报告0以外的其他内容。

答案 7 :(得分:0)

this answer的启发,我想出了一些面向对象的解决方案。

仅当您打算读取字符时才有效

您可以重写BufferedReader并实现类似这样的内容:

public class SafeBufferedReader extends BufferedReader{

    private long millisTimeout;

    ( . . . )

    @Override
    public int read(char[] cbuf, int off, int len) throws IOException {
        try {
            waitReady();
        } catch(IllegalThreadStateException e) {
            return 0;
        }
        return super.read(cbuf, off, len);
    }

    protected void waitReady() throws IllegalThreadStateException, IOException {
        if(ready()) return;
        long timeout = System.currentTimeMillis() + millisTimeout;
        while(System.currentTimeMillis() < timeout) {
            if(ready()) return;
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                break; // Should restore flag
            }
        }
        if(ready()) return; // Just in case.
        throw new IllegalThreadStateException("Read timed out");
    }
}

这是一个几乎完整的示例。

我在某些方法上返回0,您应该将其更改为-2以满足您的需要,但是我认为0更适合BufferedReader合同。没错,它只读取0个字符。 readLine方法是一个可怕的性能杀手。 如果您确实想使用readLin e,则应该创建一个全新的BufferedReader。目前,它不是线程安全的。如果有人在readLines等待一行时调用某个操作,则会产生意外的结果

我不喜欢返回-2。我会抛出一个异常,因为有些人可能只是检查int <0以考虑EOS。无论如何,这些方法声称“无法阻止”,您应该检查该语句是否确实是真的,并且不要覆盖它们。

import java.io.BufferedReader;
import java.io.IOException;
import java.io.Reader;
import java.nio.CharBuffer;
import java.util.concurrent.TimeUnit;
import java.util.stream.Stream;

/**
 * 
 * readLine
 * 
 * @author Dario
 *
 */
public class SafeBufferedReader extends BufferedReader{

    private long millisTimeout;

    private long millisInterval = 100;

    private int lookAheadLine;

    public SafeBufferedReader(Reader in, int sz, long millisTimeout) {
        super(in, sz);
        this.millisTimeout = millisTimeout;
    }

    public SafeBufferedReader(Reader in, long millisTimeout) {
        super(in);
        this.millisTimeout = millisTimeout;
    }



    /**
     * This is probably going to kill readLine performance. You should study BufferedReader and completly override the method.
     * 
     * It should mark the position, then perform its normal operation in a nonblocking way, and if it reaches the timeout then reset position and throw IllegalThreadStateException
     * 
     */
    @Override
    public String readLine() throws IOException {
        try {
            waitReadyLine();
        } catch(IllegalThreadStateException e) {
            //return null; //Null usually means EOS here, so we can't.
            throw e;
        }
        return super.readLine();
    }

    @Override
    public int read() throws IOException {
        try {
            waitReady();
        } catch(IllegalThreadStateException e) {
            return -2; // I'd throw a runtime here, as some people may just be checking if int < 0 to consider EOS
        }
        return super.read();
    }

    @Override
    public int read(char[] cbuf) throws IOException {
        try {
            waitReady();
        } catch(IllegalThreadStateException e) {
            return -2;  // I'd throw a runtime here, as some people may just be checking if int < 0 to consider EOS
        }
        return super.read(cbuf);
    }

    @Override
    public int read(char[] cbuf, int off, int len) throws IOException {
        try {
            waitReady();
        } catch(IllegalThreadStateException e) {
            return 0;
        }
        return super.read(cbuf, off, len);
    }

    @Override
    public int read(CharBuffer target) throws IOException {
        try {
            waitReady();
        } catch(IllegalThreadStateException e) {
            return 0;
        }
        return super.read(target);
    }

    @Override
    public void mark(int readAheadLimit) throws IOException {
        super.mark(readAheadLimit);
    }

    @Override
    public Stream<String> lines() {
        return super.lines();
    }

    @Override
    public void reset() throws IOException {
        super.reset();
    }

    @Override
    public long skip(long n) throws IOException {
        return super.skip(n);
    }

    public long getMillisTimeout() {
        return millisTimeout;
    }

    public void setMillisTimeout(long millisTimeout) {
        this.millisTimeout = millisTimeout;
    }

    public void setTimeout(long timeout, TimeUnit unit) {
        this.millisTimeout = TimeUnit.MILLISECONDS.convert(timeout, unit);
    }

    public long getMillisInterval() {
        return millisInterval;
    }

    public void setMillisInterval(long millisInterval) {
        this.millisInterval = millisInterval;
    }

    public void setInterval(long time, TimeUnit unit) {
        this.millisInterval = TimeUnit.MILLISECONDS.convert(time, unit);
    }

    /**
     * This is actually forcing us to read the buffer twice in order to determine a line is actually ready.
     * 
     * @throws IllegalThreadStateException
     * @throws IOException
     */
    protected void waitReadyLine() throws IllegalThreadStateException, IOException {
        long timeout = System.currentTimeMillis() + millisTimeout;
        waitReady();

        super.mark(lookAheadLine);
        try {
            while(System.currentTimeMillis() < timeout) {
                while(ready()) {
                    int charInt = super.read();
                    if(charInt==-1) return; // EOS reached
                    char character = (char) charInt;
                    if(character == '\n' || character == '\r' ) return;
                }
                try {
                    Thread.sleep(millisInterval);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt(); // Restore flag
                    break;
                }
            }
        } finally {
            super.reset();
        }
        throw new IllegalThreadStateException("readLine timed out");

    }

    protected void waitReady() throws IllegalThreadStateException, IOException {
        if(ready()) return;
        long timeout = System.currentTimeMillis() + millisTimeout;
        while(System.currentTimeMillis() < timeout) {
            if(ready()) return;
            try {
                Thread.sleep(millisInterval);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt(); // Restore flag
                break;
            }
        }
        if(ready()) return; // Just in case.
        throw new IllegalThreadStateException("read timed out");
    }

}