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。