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.
in.read()方法可能是不可中断的.
将InputStream包装在Reader或InterruptibleChannel中没有帮助,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.
in.available()返回0总是可以接受的.
in.close()方法可能会阻塞或不执行任何操作.
没有通用的方法来杀死另一个线程.
Glen Best.. 80
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.Javadoc InputStream.available():
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.当有数据时,永久返回0会导致上述条件失败.
First Caveat:InputStream的具体子类负责available()
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,javadoc available()也表示:
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.
第二个注意事项:确保在Windows中键入输入时使用回车符.
如果使用System.in,程序只会在命令shell交给它时接收输入.如果您正在使用文件重定向/管道(例如somefile> 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,数据将缓存在cmd.exe shell中.数据仅在回车(control-m或
只要用这个:
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.
或者等价地,
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);
}
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;
}
然后用这个:
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.
Ian Jones.. 65
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.
假设我有以下执行器和流:
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,然后等待5秒钟,然后写入最后一段数据并关闭流:
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);
阅读的正常方法如下.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");
哪个输出:
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,我可以控制超时,如下所示:
int readByte = 1;
// Read data with timeout
Callable readTask = new Callable() {
@Override
public Integer call() throws Exception {
return inputStream.read();
}
};
while (readByte >= 0) {
Future future = executor.submit(readTask);
readByte = future.get(1000, TimeUnit.MILLISECONDS);
if (readByte >= 0)
System.out.println("Read: " + readByte);
}
哪个输出:
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并做任何我想要的清理工作.
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.Javadoc InputStream.available():
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.当有数据时,永久返回0会导致上述条件失败.
First Caveat:InputStream的具体子类负责available()
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,javadoc available()也表示:
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.
第二个注意事项:确保在Windows中键入输入时使用回车符.
如果使用System.in,程序只会在命令shell交给它时接收输入.如果您正在使用文件重定向/管道(例如somefile> 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,数据将缓存在cmd.exe shell中.数据仅在回车(control-m或
只要用这个:
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.
或者等价地,
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);
}
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;
}
然后用这个:
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.
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.
假设我有以下执行器和流:
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,然后等待5秒钟,然后写入最后一段数据并关闭流:
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);
阅读的正常方法如下.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");
哪个输出:
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,我可以控制超时,如下所示:
int readByte = 1;
// Read data with timeout
Callable readTask = new Callable() {
@Override
public Integer call() throws Exception {
return inputStream.read();
}
};
while (readByte >= 0) {
Future future = executor.submit(readTask);
readByte = future.get(1000, TimeUnit.MILLISECONDS);
if (readByte >= 0)
System.out.println("Read: " + readByte);
}
哪个输出:
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并做任何我想要的清理工作.
如果您的InputStream由Socket支持,您可以使用setSoTimeout设置Socket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SocketTimeoutException.
只需确保在进行read()调用之前在Socket上调用setSoTimeout.
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.如果你拥有后者Socket.setSoTimeout(),HttpURLConnection.setReadTimeout()并且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尴尬的实现.
我没有使用Java NIO包中的类,但似乎它们可能在这里有所帮助.具体来说,java.nio.channels.Channels和java.nio.channels.InterruptibleChannel.
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.
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中而不是在控制台中运行程序时,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.
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