装饰模式
修饰模式（装饰模式），是面向对象编程领域中，一种动态地往一个类中添加新的行为的设计模式。就功能而言，修饰模式相比生成子类更为灵活，这样可以给某个对象而不是整个类添加一些功能。
装饰模式的UML如下所示：
装饰模式中有四个角色：
- Component 抽象构件，最基本、最核心、最原始的接口或抽象类
- ConcreteComponent 具体构件的引用
- Decorator 装饰角色， 持有对构件的引用
- ConcreteDecorator 具体装饰角色
Java IO中的装饰模式
Java IO流就是装饰模式的典型应用。
与装饰模式中角色对应的类如下：
- Component：
InputStream，OutputStream - ConcreteComponent：
FileInputStream，PipeInputStream，ByteArrayInputStream... - Decorator：
FilterInputStream，FilterOutputStream - ConcreteDecorator：
DataInputStream，BufferedInputStream，LineNumberInputStream...
FilterInputStream和FilterOutputStream做的事情很简单，只是持有了一个Stream的引用并做了代理：
package java.io;public
class FilterInputStream extends InputStream {protected volatile InputStream in;protected FilterInputStream(InputStream in) {this.in = in;}public int read() throws IOException {return in.read();}//...省略掉一些方法
}BufferedInputStream
来看下BufferedInputStream的代码（当然只是一部分）：
package java.io;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;public
class BufferedInputStream extends FilterInputStream {private static int DEFAULT_BUFFER_SIZE = 8192;protected volatile byte buf[];protected int count;protected int pos;protected int markpos = -1;protected int marklimit;public BufferedInputStream(InputStream in) {this(in, DEFAULT_BUFFER_SIZE);}public BufferedInputStream(InputStream in, int size) {super(in);if (size <= 0) {throw new IllegalArgumentException("Buffer size <= 0");}buf = new byte[size];}private void fill() throws IOException {byte[] buffer = getBufIfOpen();if (markpos < 0)pos = 0; /* no mark: throw away the buffer */else if (pos >= buffer.length) /* no room left in buffer */if (markpos > 0) { /* can throw away early part of the buffer */int sz = pos - markpos;System.arraycopy(buffer, markpos, buffer, 0, sz);pos = sz;markpos = 0;} else if (buffer.length >= marklimit) {markpos = -1; /* buffer got too big, invalidate mark */pos = 0; /* drop buffer contents */} else if (buffer.length >= MAX_BUFFER_SIZE) {throw new OutOfMemoryError("Required array size too large");} else { /* grow buffer */int nsz = (pos <= MAX_BUFFER_SIZE - pos) ?pos * 2 : MAX_BUFFER_SIZE;if (nsz > marklimit)nsz = marklimit;byte nbuf[] = new byte[nsz];System.arraycopy(buffer, 0, nbuf, 0, pos);if (!bufUpdater.compareAndSet(this, buffer, nbuf)) {throw new IOException("Stream closed");}buffer = nbuf;}count = pos;int n = getInIfOpen().read(buffer, pos, buffer.length - pos);if (n > 0)count = n + pos;}public synchronized int read() throws IOException {if (pos >= count) {fill();if (pos >= count)return -1;}return getBufIfOpen()[pos++] & 0xff;}public synchronized int read(byte b[], int off, int len)throws IOException{getBufIfOpen(); // Check for closed streamif ((off | len | (off + len) | (b.length - (off + len))) < 0) {throw new IndexOutOfBoundsException();} else if (len == 0) {return 0;}int n = 0;for (;;) {int nread = read1(b, off + n, len - n);if (nread <= 0)return (n == 0) ? nread : n;n += nread;if (n >= len)return n;// if not closed but no bytes available, returnInputStream input = in;if (input != null && input.available() <= 0)return n;}}
}BufferedInputStream中有一个byte数组作为缓存，存放从制定的InputStream中读出的字节；- 它的read放回会先查看buf数组中是否还有可读的字节，如果没有就先调用一次
fill()方法从指定的stream中读取字节到buf数组中（或者直接去stream中读取足够的字节，再调用fill()方法）； BufferedInputStream支持mark，fill()方法会在buf中保留markpos到pos的这个区间内（包括markpos，不包括pos）的字节，当然前提是markpos有效；- 当markpos为0，buf数组中没有空间，buf数组的长度小于等于pos并小于 marklimit和MAX_BUFFER_SIZE，buf将被一个长度为 marklimit、MAX_BUFFER_SIZE和 2 * p中较小值的数组代替（原数组中的字节会被拷贝）。
关于mark的问题
BufferedInputStream的mark()方法是这样的:
/*** See the general contract of the <code>mark</code>* method of <code>InputStream</code>.** @param readlimit the maximum limit of bytes that can be read before* the mark position becomes invalid.* @see java.io.BufferedInputStream#reset()*/public synchronized void mark(int readlimit) {marklimit = readlimit;markpos = pos;}按照doc的意思，markpos应该在读取的字节数超过了readlimit的时候就应该失效。
但是实际上，只有fill方法中的这一段代码让markpos失效了：
if (buffer.length >= marklimit) {markpos = -1; /* buffer got too big, invalidate mark */pos = 0; /* drop buffer contents */
}也就是说，如果marklimit小于buf数组长度，markpos是不会失效的：
public static void main(String[] args) throws IOException {byte[] bytes = new byte[]{0, 1, 2, 3};ByteArrayInputStream in = new ByteArrayInputStream(bytes);BufferedInputStream bin = new BufferedInputStream(in);//如果制定了size为1，这段代码将会报错//BufferedInputStream bin = new BufferedInputStream(in, 1);bin.mark(1);bin.read();bin.read();bin.reset();
}当然，之前也有提到，如果markpos为0， buf是有可能扩容的。
参考资料
JDK8源码
《设计模式之禅》第二版
修饰模式