आप PushbackInputStream के साथ इनपुट स्ट्रीम लपेट सकते हैं। PushbackInputStream अपठित करने की अनुमति देता है (" राइट बैक ") बाइट्स जो पहले से ही पढ़े गए थे, इसलिए आप इस तरह कर सकते हैं:
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();
}
}
कृपया ध्यान दें कि PushbackInputStream बाइट्स के आंतरिक बफर को संग्रहीत करता है इसलिए यह वास्तव में मेमोरी में एक बफर बनाता है जो बाइट्स को "वापस लिखा गया" रखता है।
इस दृष्टिकोण को जानने के बाद हम आगे जा सकते हैं और इसे FilterInputStream के साथ जोड़ सकते हैं। FilterInputStream एक प्रतिनिधि के रूप में मूल इनपुट स्ट्रीम संग्रहीत करता है। यह नई कक्षा परिभाषा बनाने की अनुमति देता है जो मूल डेटा को " अपठित " करने की अनुमति देता है । इस वर्ग की परिभाषा निम्नलिखित है:
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);
}
}
}
इस वर्ग के दो तरीके हैं। मौजूदा बफ़र में पढ़ने के लिए एक (डिफिनिटेशन public int read(byte b[], int off, int len)इनपुटस्ट्रीम क्लास की कॉलिंग के अनुरूप है )। दूसरा जो नया बफर लौटाता है (यह अधिक प्रभावी हो सकता है यदि बफर का आकार अज्ञात है)।
अब देखते हैं कि हमारी कक्षा क्या है:
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
}
}