装饰模式和Java IO

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:InputStreamOutputStream
  • ConcreteComponent: FileInputStreamPipeInputStreamByteArrayInputStream ...
  • Decorator:FilterInputStreamFilterOutputStream
  • ConcreteDecorator:DataInputStreamBufferedInputStreamLineNumberInputStream...

FilterInputStreamFilterOutputStream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 stream
        if ((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, return
            InputStream 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

BufferedInputStreammark()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

posted @ 2018-06-04 01:00 -.-| 阅读(...) 评论(...) 编辑 收藏