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longxitian:
https://www.cnblogs.com/jeffen/ ...
万恶的Mybatis的EnumTypeHandler -
asialee:
ddnzero 写道博主请问FileUtils这个类是哪个包的 ...
使用mockftpserver进行ftp测试 -
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感觉LogManager打开了所有的LogSegment(文件 ...
jafka学习之LogManager
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); } }
发表评论
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使用commons-net对ftp文件上传下载
2012-08-01 18:38 2563项目中由于要使用到ftp服务,虽然之前对edtFT ... -
commons-io之inputstream学习
2010-08-20 19:41 2583ProxyInputStream类的学习 ... -
commons-io之WildcardFileFilter的实现
2010-08-02 11:35 2475上次这个是最后一个FileFilter,没 ... -
commons-io之filefilter学习
2010-07-27 10:23 50811. IOFileFilter接口 这个接口就是 ... -
common-io之Comparator阅读
2010-07-17 00:17 9941. 首先我们来查看AbstractFileCompa ... -
一款文件上传信息即时同步刷新的代码的学习
2010-07-06 18:04 1292对下面链接提供的文件上传的代码的学习。 http://mao ... -
FileUpload之FileItem
2010-07-05 18:17 22690FileItem类主要是封装了一个File Item ... -
commons-io之ThresholdingOutputStream 和 DeferredFileOutputStream
2010-06-26 19:44 25491. ThresholdingOutputStream 这个 ... -
Digester 1.1 单元测试之RuleTestCase
2010-02-03 23:32 1329前面还记得有前辈说过看一个开源项目在不懂的情况下要去 ... -
Digester 1.1 源码阅读
2010-02-03 20:36 1504前一段时间我们阅读了Digester的最初始的版本1 ... -
Digester 1.0 源码阅读
2010-02-01 20:15 1594近来在学习tomcat ... -
Digester学习
2010-01-26 00:45 1902近来在学习tomcat的源码,其中有个解析XML的 ...
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