Você pode agrupar o fluxo de entrada com PushbackInputStream. PushbackInputStream permite não ler (" escrever de volta ") bytes que já foram lidos, para que você possa fazer assim:
public class StreamTest {
public static void main(String[] args) throws IOException {
byte[] bytes = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
InputStream originalStream = new ByteArrayInputStream(bytes);
byte[] readBytes = getBytes(originalStream, 3);
printBytes(readBytes); // prints: 1 2 3
readBytes = getBytes(originalStream, 3);
printBytes(readBytes); // prints: 4 5 6
// now let's wrap it with PushBackInputStream
originalStream = new ByteArrayInputStream(bytes);
InputStream wrappedStream = new PushbackInputStream(originalStream, 10); // 10 means that maximnum 10 characters can be "written back" to the stream
readBytes = getBytes(wrappedStream, 3);
printBytes(readBytes); // prints 1 2 3
((PushbackInputStream) wrappedStream).unread(readBytes, 0, readBytes.length);
readBytes = getBytes(wrappedStream, 3);
printBytes(readBytes); // prints 1 2 3
}
private static byte[] getBytes(InputStream is, int howManyBytes) throws IOException {
System.out.print("Reading stream: ");
byte[] buf = new byte[howManyBytes];
int next = 0;
for (int i = 0; i < howManyBytes; i++) {
next = is.read();
if (next > 0) {
buf[i] = (byte) next;
}
}
return buf;
}
private static void printBytes(byte[] buffer) throws IOException {
System.out.print("Reading stream: ");
for (int i = 0; i < buffer.length; i++) {
System.out.print(buffer[i] + " ");
}
System.out.println();
}
}
Observe que PushbackInputStream armazena buffer interno de bytes para que ele realmente crie um buffer na memória que retenha os bytes "gravados de volta".
Conhecendo essa abordagem, podemos ir além e combiná-la com o FilterInputStream. FilterInputStream armazena o fluxo de entrada original como um representante. Isso permite criar uma nova definição de classe que permite " não ler " dados originais automaticamente. A definição desta classe é a seguinte:
public class TryReadInputStream extends FilterInputStream {
private final int maxPushbackBufferSize;
/**
* Creates a <code>FilterInputStream</code>
* by assigning the argument <code>in</code>
* to the field <code>this.in</code> so as
* to remember it for later use.
*
* @param in the underlying input stream, or <code>null</code> if
* this instance is to be created without an underlying stream.
*/
public TryReadInputStream(InputStream in, int maxPushbackBufferSize) {
super(new PushbackInputStream(in, maxPushbackBufferSize));
this.maxPushbackBufferSize = maxPushbackBufferSize;
}
/**
* Reads from input stream the <code>length</code> of bytes to given buffer. The read bytes are still avilable
* in the stream
*
* @param buffer the destination buffer to which read the data
* @param offset the start offset in the destination <code>buffer</code>
* @aram length how many bytes to read from the stream to buff. Length needs to be less than
* <code>maxPushbackBufferSize</code> or IOException will be thrown
*
* @return number of bytes read
* @throws java.io.IOException in case length is
*/
public int tryRead(byte[] buffer, int offset, int length) throws IOException {
validateMaxLength(length);
// NOTE: below reading byte by byte instead of "int bytesRead = is.read(firstBytes, 0, maxBytesOfResponseToLog);"
// because read() guarantees to read a byte
int bytesRead = 0;
int nextByte = 0;
for (int i = 0; (i < length) && (nextByte >= 0); i++) {
nextByte = read();
if (nextByte >= 0) {
buffer[offset + bytesRead++] = (byte) nextByte;
}
}
if (bytesRead > 0) {
((PushbackInputStream) in).unread(buffer, offset, bytesRead);
}
return bytesRead;
}
public byte[] tryRead(int maxBytesToRead) throws IOException {
validateMaxLength(maxBytesToRead);
ByteArrayOutputStream baos = new ByteArrayOutputStream(); // as ByteArrayOutputStream to dynamically allocate internal bytes array instead of allocating possibly large buffer (if maxBytesToRead is large)
// NOTE: below reading byte by byte instead of "int bytesRead = is.read(firstBytes, 0, maxBytesOfResponseToLog);"
// because read() guarantees to read a byte
int nextByte = 0;
for (int i = 0; (i < maxBytesToRead) && (nextByte >= 0); i++) {
nextByte = read();
if (nextByte >= 0) {
baos.write((byte) nextByte);
}
}
byte[] buffer = baos.toByteArray();
if (buffer.length > 0) {
((PushbackInputStream) in).unread(buffer, 0, buffer.length);
}
return buffer;
}
private void validateMaxLength(int length) throws IOException {
if (length > maxPushbackBufferSize) {
throw new IOException(
"Trying to read more bytes than maxBytesToRead. Max bytes: " + maxPushbackBufferSize + ". Trying to read: " +
length);
}
}
}
Esta classe tem dois métodos. Um para ler no buffer existente (a definição é análoga à chamadapublic int read(byte b[], int off, int len) da classe InputStream). Segundo, que retorna um novo buffer (isso pode ser mais eficaz se o tamanho do buffer para leitura for desconhecido).
Agora vamos ver nossa classe em ação:
public class StreamTest2 {
public static void main(String[] args) throws IOException {
byte[] bytes = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
InputStream originalStream = new ByteArrayInputStream(bytes);
byte[] readBytes = getBytes(originalStream, 3);
printBytes(readBytes); // prints: 1 2 3
readBytes = getBytes(originalStream, 3);
printBytes(readBytes); // prints: 4 5 6
// now let's use our TryReadInputStream
originalStream = new ByteArrayInputStream(bytes);
InputStream wrappedStream = new TryReadInputStream(originalStream, 10);
readBytes = ((TryReadInputStream) wrappedStream).tryRead(3); // NOTE: no manual call to "unread"(!) because TryReadInputStream handles this internally
printBytes(readBytes); // prints 1 2 3
readBytes = ((TryReadInputStream) wrappedStream).tryRead(3);
printBytes(readBytes); // prints 1 2 3
readBytes = ((TryReadInputStream) wrappedStream).tryRead(3);
printBytes(readBytes); // prints 1 2 3
// we can also call normal read which will actually read the bytes without "writing them back"
readBytes = getBytes(wrappedStream, 3);
printBytes(readBytes); // prints 1 2 3
readBytes = getBytes(wrappedStream, 3);
printBytes(readBytes); // prints 4 5 6
readBytes = ((TryReadInputStream) wrappedStream).tryRead(3); // now we can try read next bytes
printBytes(readBytes); // prints 7 8 9
readBytes = ((TryReadInputStream) wrappedStream).tryRead(3);
printBytes(readBytes); // prints 7 8 9
}
}