Java IO字节输入流常见类进行分析（二）

一、FileInputStream
从文件系统中读取一个文件转换成的字节
数据结构：

public class FileInputStream extends InputStream{    /* File Descriptor - handle to the open file */    private final FileDescriptor fd;    /* The path of the referenced file (null if the stream is created with a file descriptor) */    private final String path;//文件的路径    private FileChannel channel = null;//与文件相连的通道    private final Object closeLock = new Object();    private volatile boolean closed = false;    private static final ThreadLocal<Boolean> runningFinalize =        new ThreadLocal<>();   //...   }

重载的构造方法

//通过文件来创建public FileInputStream(File file) throws FileNotFoundException {        String name = (file != null ? file.getPath() : null);        SecurityManager security = System.getSecurityManager();        if (security != null) {            security.checkRead(name);//检查权限，例如读权限        }        if (name == null) {            throw new NullPointerException();        }        if (file.isInvalid()) {            throw new FileNotFoundException("Invalid file path");        }        fd = new FileDescriptor();        fd.incrementAndGetUseCount();//增加用户使用的次数        this.path = name;        open(name);//打开文件    }    //通过文件描述器来创建文件输入流    public FileInputStream(FileDescriptor fdObj) {        SecurityManager security = System.getSecurityManager();        if (fdObj == null) {            throw new NullPointerException();        }        if (security != null) {            security.checkRead(fdObj);        }        fd = fdObj;        path = null;        /*         * FileDescriptor is being shared by streams.         * Ensure that it's GC'ed only when all the streams/channels are done         * using it.         */        fd.incrementAndGetUseCount();    }    //通过文件的路径名来创建输入流    public FileInputStream(String name) throws FileNotFoundException {        this(name != null ? new File(name) : null);    }

FileDescriptor是FileInputStream与文件的连接桥梁，它的创建代表着文件的打开或者socket的打开。
部分方法：
close方法

public void close() throws IOException {        synchronized (closeLock) {//同步锁            if (closed) {                return;            }            closed = true;        }        if (channel != null) {           //减少File Descriptor的数量           fd.decrementAndGetUseCount();           channel.close();        }        /*         *减少File Descriptor的数量         */        int useCount = fd.decrementAndGetUseCount();        /*         * 如果File Descriptor的被其他流使用，那就将不会关闭         */        if ((useCount <= 0) || !isRunningFinalize()) {            close0();        }    }

getChannel() 增加FileDescriptor的数量

public FileChannel getChannel(){    synchronized (this) {            if (channel == null) {         channel = FileChannelImpl.open(fd, path, true, false, this);                //增加FileDescriptor                fd.incrementAndGetUseCount();            }            return channel;        }}

二、SocketInputStream
继承与FileInputStream，注意的是该类没有被public修饰。看一下类的结构

class SocketInputStream extends FileInputStream{    private boolean eof;//结束标志    private AbstractPlainSocketImpl impl = null;//socket现实抽象类    private byte temp[];//缓冲数据    private Socket socket = null;}

从这里可以看出，File IO和Socket IO非常的相似。
三、ObjectInputStream
一个反序列化之前写入到ObjectOutputSteam中的原生类型数据和对象类型。ObjectInputStream确保了在图中流所匹配在JVM中的java类。