本文详细解析了ByteArrayOutputStream的工作原理及内部实现机制，包括构造函数、关键方法如write、reset及toByteArray等的功能与流程。
首先我们来分析它里面的的实例变量：
buffers: 可以看成是一个buffer仓库，里面放的是已经读取的所有数据
currentBufferIndex: 就是正在使用的buffer的index
count： 用来存放buffers里面的所有的字节数
currentBuffer: 就是当前的使用buffer，这个比较好理解。
filledBufferSum: 这个起初的时候我特别不理解，后来我理解，主要是用了保存所有的满buffer的字节数的总和。
举个例子：
起始的时候第一个buffer的大小为32，它的filledBufferSum为0，count为0，然后我们给当前的buffer放入5个字节的数据，现在count为5， 下一次我们count - filedBufferSum 就是我们下一次要存储的buffer的指针，比如我们要放25个字节，那么现在count就变成30了，filledBufferSum仍然是0，我们再放入3个字节，现在count变成33，比buffer的初始大小大了，就扩容，新建一个buffer，把老的buffer放到buffers里面，然后filledBufferSum就变成32了，把扩容后剩余的1个字节放到新申请的buffer里面，下一次比如我们想再放入10个字节的数据，count是33，filedBufferSum是32,我们存放的指针应该是1，因为0字节我们存放了上次扩容后的剩余的字节数。
1. 首先来看一下构造函数：
public ByteArrayOutputStream() {
this(1024);
}
public ByteArrayOutputStream(int size) {
if (size < 0) {
throw new IllegalArgumentException(
"Negative initial size: " + size);
}
needNewBuffer(size);
}
一个是无参数的时候创建一个大小为1024的buffer，一个是根据用户输入的大小创建buffer，这个都比较好理解，关键是needNewBuffer函数，这个放到下面进行讲解。
2. 下面来看一下needNewBuffer函数，这个是这个类的灵魂，我感觉
private void needNewBuffer(int newcount) {
if (currentBufferIndex < buffers.size() - 1) {
//Recycling old buffer
filledBufferSum += currentBuffer.length;
currentBufferIndex++;
currentBuffer = getBuffer(currentBufferIndex);
} else {
//Creating new buffer
int newBufferSize;
if (currentBuffer == null) {
newBufferSize = newcount;
filledBufferSum = 0;
} else {
newBufferSize = Math.max(
currentBuffer.length << 1,
newcount - filledBufferSum);
filledBufferSum += currentBuffer.length;
}
currentBufferIndex++;
currentBuffer = new byte[newBufferSize];
buffers.add(currentBuffer);
}
}首先我们为了好理解期间，我们先来讲else分支，if分支我们放到reset函数里讲解。
这个就是我们传统意义上的创建新的buffer。这个里面如果currentBuffer是null的话，
就初始化一个，这个分支是在创建的时候会走这个分支，下一个分支是将当前的buffer
的length乘以2和所需要的大小进行比较取最大值来当成新的buffer的大小，
然后把filledBufferSum的值进行更改。下面几行代码是来真正创建buffer的地方，
并把他加到buffers里面去。
3. 下面我们来看一下write函数
public void write(byte[] b, int off, int len) {
if ((off < 0)
|| (off > b.length)
|| (len < 0)
|| ((off + len) > b.length)
|| ((off + len) < 0)) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return;
}
synchronized (this) {
int newcount = count + len;
int remaining = len;
int inBufferPos = count - filledBufferSum;
while (remaining > 0) {
int part = Math.min(remaining, currentBuffer.length - inBufferPos);
System.arraycopy(b, off + len - remaining, currentBuffer, inBufferPos, part);
remaining -= part;
if (remaining > 0) {
needNewBuffer(newcount);
inBufferPos = 0;
}
}
count = newcount;
}
}
首先是一些检验，放在数组越界，这些判断是和父类里面的判断是相同的。
下面是才真正的实现写操作。 首先来计算新的count，并将要写的字节数当成初始的remaining，并来计算这次要写的指针位置， 就是上次的总大小减去已经存放满的buffer里面的字节数。
在这里将remaining和当前所剩余的空间做了一个比较，取最小值。然后做数组拷贝动作。然后判断是不是已经完全写完，如果没写完的话就是分配空间了，然后执行分配空间动作，最后在循环的写入到buffer里面。
4. 下面我们在巩固一下，理解一下write函数
/**
* Write a byte to byte array.
* @param b the byte to write
*/
public synchronized void write(int b) {
int inBufferPos = count - filledBufferSum;
if (inBufferPos == currentBuffer.length) {
needNewBuffer(count + 1);
inBufferPos = 0;
}
currentBuffer[inBufferPos] = (byte) b;
count++;
}只要理解了上面的，这个函数就特别好理解，首先是得到这次应该写入的指针位置，如果发现没有空间的话进行分配新的空间，然后把数据写到buffer里面，并累加写入的数据总数。
5. write函数的研究
public synchronized int write(InputStream in) throws IOException {
int readCount = 0;
int inBufferPos = count - filledBufferSum;
int n = in.read(currentBuffer, inBufferPos, currentBuffer.length - inBufferPos);
while (n != -1) {
readCount += n;
inBufferPos += n;
count += n;
if (inBufferPos == currentBuffer.length) {
needNewBuffer(currentBuffer.length);
inBufferPos = 0;
}
n = in.read(currentBuffer, inBufferPos, currentBuffer.length - inBufferPos);
}
return readCount;
}这个函数其实也比较好理解，主要是计算应该写入的指针位置，然后从InputStream里面读到currentBuffer里面，从inBufferPos开始，总共读最大剩余长度个字节，如果读到的不为-1，则变化当前的指针和readCount，然后如果当前的指针和currentBuffer的length相等时，则进行扩充缓冲区，然后进行循环，直到读完。
6. reset函数
public synchronized void reset() {
count = 0;
filledBufferSum = 0;
currentBufferIndex = 0;
currentBuffer = getBuffer(currentBufferIndex);
}我起始得时候不理解，现在理解了。比如我们首先对第一个文件进行了写操作，然后缓冲区并没有释放，我们要读下一个文件，那么我们现在肯定不能再去申请空间了，要对上一次的缓冲区进行复用，这里就先将一些状态变量清0，然后将当前的buffer设置为第0个buffer，这样下一次操作的时候就可以使用上次分配的buffer了。为了把问题说明白，我们再来看一下
needNewBuffer的if分支，
if (currentBufferIndex < buffers.size() - 1) {
//Recycling old buffer
filledBufferSum += currentBuffer.length;
currentBufferIndex++;
currentBuffer = getBuffer(currentBufferIndex);这个里面，我们可以看到调用了这个函数后，currentBufferIndex是0，如果buffers还有其他缓冲区的话就将当前的bufferIndex加1，并返回下一个缓冲区，这样感觉效率比较高，到这里我们估计你应该彻底理解这个里面的流程了。
7. 下面我们来看一下writeTo函数的实现
public synchronized void writeTo(OutputStream out) throws IOException {
int remaining = count;
for (int i = 0; i < buffers.size(); i++) {
byte[] buf = getBuffer(i);
int c = Math.min(buf.length, remaining);
out.write(buf, 0, c);
remaining -= c;
if (remaining == 0) {
break;
}
}
}这个函数主要是实现了把buf中的数据直接写到用户指定的OutputStream里面。这个里面和JDK里面的实现的唯一的差别就是这里会循环使得buffers中的数据会全部写到指定的OutputStream里面。
8. 下面我们来发一下toByteArray的实现。
public synchronized byte[] toByteArray() {
int remaining = count;
if (remaining == 0) {
return EMPTY_BYTE_ARRAY;
}
byte newbuf[] = new byte[remaining];
int pos = 0;
for (int i = 0; i < buffers.size(); i++) {
byte[] buf = getBuffer(i);
int c = Math.min(buf.length, remaining);
System.arraycopy(buf, 0, newbuf, pos, c);
pos += c;
remaining -= c;
if (remaining == 0) {
break;
}
}
return newbuf;
}
这个函数很好理解，就是创建一个count大小的byte数组，然后循环buffers，将每一个缓冲区中的数据都copy到将要返回的字节数组里面。
下面完整的程序：
public class ByteArrayOutputStream extends OutputStream {
/** A singleton empty byte array. */
private static final byte[] EMPTY_BYTE_ARRAY = new byte[0];
/** The list of buffers, which grows and never reduces. */
private final List<byte[]> buffers = new ArrayList<byte[]>();
/** The index of the current buffer. */
private int currentBufferIndex;
/** The total count of bytes in all the filled buffers. */
private int filledBufferSum;
/** The current buffer. */
private byte[] currentBuffer;
/** The total count of bytes written. */
private int count;
/**
* Creates a new byte array output stream. The buffer capacity is
* initially 1024 bytes, though its size increases if necessary.
*/
public ByteArrayOutputStream() {
this(1024);
}
/**
* Creates a new byte array output stream, with a buffer capacity of
* the specified size, in bytes.
*
* @param size the initial size
* @throws IllegalArgumentException if size is negative
*/
public ByteArrayOutputStream(int size) {
if (size < 0) {
throw new IllegalArgumentException(
"Negative initial size: " + size);
}
needNewBuffer(size);
}
/**
* Makes a new buffer available either by allocating
* a new one or re-cycling an existing one.
*
* @param newcount the size of the buffer if one is created
*/
private void needNewBuffer(int newcount) {
if (currentBufferIndex < buffers.size() - 1) {
//Recycling old buffer
filledBufferSum += currentBuffer.length;
currentBufferIndex++;
currentBuffer = buffers.get(currentBufferIndex);
} else {
//Creating new buffer
int newBufferSize;
if (currentBuffer == null) {
newBufferSize = newcount;
filledBufferSum = 0;
} else {
newBufferSize = Math.max(
currentBuffer.length << 1,
newcount - filledBufferSum);
filledBufferSum += currentBuffer.length;
}
currentBufferIndex++;
currentBuffer = new byte[newBufferSize];
buffers.add(currentBuffer);
}
}
/**
* Write the bytes to byte array.
* @param b the bytes to write
* @param off The start offset
* @param len The number of bytes to write
*/
@Override
public void write(byte[] b, int off, int len) {
if ((off < 0)
|| (off > b.length)
|| (len < 0)
|| ((off + len) > b.length)
|| ((off + len) < 0)) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return;
}
synchronized (this) {
int newcount = count + len;
int remaining = len;
int inBufferPos = count - filledBufferSum;
while (remaining > 0) {
int part = Math.min(remaining, currentBuffer.length - inBufferPos);
System.arraycopy(b, off + len - remaining, currentBuffer, inBufferPos, part);
remaining -= part;
if (remaining > 0) {
needNewBuffer(newcount);
inBufferPos = 0;
}
}
count = newcount;
}
}
/**
* Write a byte to byte array.
* @param b the byte to write
*/
@Override
public synchronized void write(int b) {
int inBufferPos = count - filledBufferSum;
if (inBufferPos == currentBuffer.length) {
needNewBuffer(count + 1);
inBufferPos = 0;
}
currentBuffer[inBufferPos] = (byte) b;
count++;
}
/**
* Writes the entire contents of the specified input stream to this
* byte stream. Bytes from the input stream are read directly into the
* internal buffers of this streams.
*
* @param in the input stream to read from
* @return total number of bytes read from the input stream
* (and written to this stream)
* @throws IOException if an I/O error occurs while reading the input stream
* @since Commons IO 1.4
*/
public synchronized int write(InputStream in) throws IOException {
int readCount = 0;
int inBufferPos = count - filledBufferSum;
int n = in.read(currentBuffer, inBufferPos, currentBuffer.length - inBufferPos);
while (n != -1) {
readCount += n;
inBufferPos += n;
count += n;
if (inBufferPos == currentBuffer.length) {
needNewBuffer(currentBuffer.length);
inBufferPos = 0;
}
n = in.read(currentBuffer, inBufferPos, currentBuffer.length - inBufferPos);
}
return readCount;
}
/**
* Return the current size of the byte array.
* @return the current size of the byte array
*/
public synchronized int size() {
return count;
}
/**
* Closing a <tt>ByteArrayOutputStream</tt> has no effect. The methods in
* this class can be called after the stream has been closed without
* generating an <tt>IOException</tt>.
*
* @throws IOException never (this method should not declare this exception
* but it has to now due to backwards compatability)
*/
@Override
public void close() throws IOException {
//nop
}
/**
* @see java.io.ByteArrayOutputStream#reset()
*/
public synchronized void reset() {
count = 0;
filledBufferSum = 0;
currentBufferIndex = 0;
currentBuffer = buffers.get(currentBufferIndex);
}
/**
* Writes the entire contents of this byte stream to the
* specified output stream.
*
* @param out the output stream to write to
* @throws IOException if an I/O error occurs, such as if the stream is closed
* @see java.io.ByteArrayOutputStream#writeTo(OutputStream)
*/
public synchronized void writeTo(OutputStream out) throws IOException {
int remaining = count;
for (byte[] buf : buffers) {
int c = Math.min(buf.length, remaining);
out.write(buf, 0, c);
remaining -= c;
if (remaining == 0) {
break;
}
}
}
/**
* Fetches entire contents of an <code>InputStream</code> and represent
* same data as result InputStream.
* <p>
* This method is useful where,
* <ul>
* <li>Source InputStream is slow.</li>
* <li>It has network resources associated, so we cannot keep it open for
* long time.</li>
* <li>It has network timeout associated.</li>
* </ul>
* It can be used in favor of {@link #toByteArray()}, since it
* avoids unnecessary allocation and copy of byte[].<br>
* This method buffers the input internally, so there is no need to use a
* <code>BufferedInputStream</code>.
*
* @param input Stream to be fully buffered.
* @return A fully buffered stream.
* @throws IOException if an I/O error occurs
*/
public static InputStream toBufferedInputStream(InputStream input)
throws IOException {
ByteArrayOutputStream output = new ByteArrayOutputStream();
output.write(input);
return output.toBufferedInputStream();
}
/**
* Gets the current contents of this byte stream as a Input Stream. The
* returned stream is backed by buffers of <code>this</code> stream,
* avoiding memory allocation and copy, thus saving space and time.<br>
*
* @return the current contents of this output stream.
* @see java.io.ByteArrayOutputStream#toByteArray()
* @see #reset()
* @since Commons IO 2.0
*/
private InputStream toBufferedInputStream() {
int remaining = count;
if (remaining == 0) {
return new ClosedInputStream();
}
List<ByteArrayInputStream> list = new ArrayList<ByteArrayInputStream>(buffers.size());
for (byte[] buf : buffers) {
int c = Math.min(buf.length, remaining);
list.add(new ByteArrayInputStream(buf, 0, c));
remaining -= c;
if (remaining == 0) {
break;
}
}
return new SequenceInputStream(Collections.enumeration(list));
}
/**
* Gets the curent contents of this byte stream as a byte array.
* The result is independent of this stream.
*
* @return the current contents of this output stream, as a byte array
* @see java.io.ByteArrayOutputStream#toByteArray()
*/
public synchronized byte[] toByteArray() {
int remaining = count;
if (remaining == 0) {
return EMPTY_BYTE_ARRAY;
}
byte newbuf[] = new byte[remaining];
int pos = 0;
for (byte[] buf : buffers) {
int c = Math.min(buf.length, remaining);
System.arraycopy(buf, 0, newbuf, pos, c);
pos += c;
remaining -= c;
if (remaining == 0) {
break;
}
}
return newbuf;
}
/**
* Gets the curent contents of this byte stream as a string.
* @return the contents of the byte array as a String
* @see java.io.ByteArrayOutputStream#toString()
*/
@Override
public String toString() {
return new String(toByteArray());
}
/**
* Gets the curent contents of this byte stream as a string
* using the specified encoding.
*
* @param enc the name of the character encoding
* @return the string converted from the byte array
* @throws UnsupportedEncodingException if the encoding is not supported
* @see java.io.ByteArrayOutputStream#toString(String)
*/
public String toString(String enc) throws UnsupportedEncodingException {
return new String(toByteArray(), enc);
}
}
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