FileInputStream和FileOutputStream详解

一.引子
　　文件，作为常见的数据源。关于操作文件的字节流就是 FileInputStream & FileOutputStream。它们是Basic IO字节流中重要的实现类。
二、FileInputStream源码分析

　　FileInputStream源码如下：

/**
 * FileInputStream 从文件系统的文件中获取输入字节流。文件取决于主机系统。
 *  比如读取图片等的原始字节流。如果读取字符流，考虑使用 FiLeReader。
 */
public class FileInputStream extends InputStream{
    /* 文件描述符类---此处用于打开文件的句柄 */
    private final FileDescriptor fd;
    /* 引用文件的路径 */
    private final String path; 
    /* 文件通道，NIO部分 */
    private FileChannel channel = null;
    private final Object closeLock = new Object();
    private volatile boolean closed = false;
 
    private static final ThreadLocal<Boolean> runningFinalize =
        new ThreadLocal<>();
 
    private static boolean isRunningFinalize() {
        Boolean val;
        if ((val = runningFinalize.get()) != null)
            return val.booleanValue();
        return false;
    }
 
    /* 通过文件路径名来创建FileInputStream */
    public FileInputStream(String name) throws FileNotFoundException {
        this(name != null ? new File(name) : null);
    }
 
    /* 通过文件来创建FileInputStream */
    public FileInputStream(File file) throws FileNotFoundException {
        String name = (file != null ? file.getPath() : null);
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkRead(name);
        }
        if (name == null) {
            throw new NullPointerException();
        }
        if (file.isInvalid()) {
            throw new FileNotFoundException("Invalid file path");
        }
        fd = new FileDescriptor();
        fd.incrementAndGetUseCount();
        this.path = name;
        open(name);
    }
 
    /* 通过文件描述符类来创建FileInputStream */
    public FileInputStream(FileDescriptor fdObj) {
        SecurityManager security = System.getSecurityManager();
        if (fdObj == null) {
            throw new NullPointerException();
        }
        if (security != null) {
            security.checkRead(fdObj);
        }
        fd = fdObj;
        path = null;
        fd.incrementAndGetUseCount();
    }
 
    /* 打开文件，为了下一步读取文件内容。native方法 */
    private native void open(String name) throws FileNotFoundException;
 
    /* 从此输入流中读取一个数据字节 */
    public int read() throws IOException {
        Object traceContext = IoTrace.fileReadBegin(path);
        int b = 0;
        try {
            b = read0();
        } finally {
            IoTrace.fileReadEnd(traceContext, b == -1 ? 0 : 1);
        }
        return b;
    }
 
    /* 从此输入流中读取一个数据字节。native方法 */
    private native int read0() throws IOException;
 
    /* 从此输入流中读取多个字节到byte数组中。native方法 */
    private native int readBytes(byte b[], int off, int len) throws IOException;
 
    /* 从此输入流中读取多个字节到byte数组中。 */
    public int read(byte b[]) throws IOException {
        Object traceContext = IoTrace.fileReadBegin(path);
        int bytesRead = 0;
        try {
            bytesRead = readBytes(b, 0, b.length);
        } finally {
            IoTrace.fileReadEnd(traceContext, bytesRead == -1 ? 0 : bytesRead);
        }
        return bytesRead;
    }
 
    /* 从此输入流中读取最多len个字节到byte数组中。 */
    public int read(byte b[], int off, int len) throws IOException {
        Object traceContext = IoTrace.fileReadBegin(path);
        int bytesRead = 0;
        try {
            bytesRead = readBytes(b, off, len);
        } finally {
            IoTrace.fileReadEnd(traceContext, bytesRead == -1 ? 0 : bytesRead);
        }
        return bytesRead;
    }
 
     
    public native long skip(long n) throws IOException;
 
    /* 返回下一次对此输入流调用的方法可以不受阻塞地从此输入流读取（或跳过）的估计剩余字节数。 */
    public native int available() throws IOException;
 
    /* 关闭此文件输入流并释放与此流有关的所有系统资源。 */
    public void close() throws IOException {
        synchronized (closeLock) {
            if (closed) {
                return;
            }
            closed = true;
        }
        if (channel != null) {
           fd.decrementAndGetUseCount();
           channel.close();
        }
 
        int useCount = fd.decrementAndGetUseCount();
 
        if ((useCount <= 0) || !isRunningFinalize()) {
            close0();
        }
    }
 
    public final FileDescriptor getFD() throws IOException {
        if (fd != null) return fd;
        throw new IOException();
    }
 
    /* 获取此文件输入流的唯一FileChannel对象 */
    publicFileChannel getChannel() {
        synchronized(this) {
            if(channel == null) {
                channel = FileChannelImpl.open(fd, path, true, false, this);
                fd.incrementAndGetUseCount();
            }
            returnchannel;
        }
    }
 
    privatestaticnativevoidinitIDs();
 
    privatenativevoidclose0() throwsIOException;
 
    static{
        initIDs();
    }
 
    protected void finalize() throws IOException {
        if((fd != null) &&  (fd != FileDescriptor.in)) {
            runningFinalize.set(Boolean.TRUE);
            try{
                close();
            } finally{
                runningFinalize.set(Boolean.FALSE);
            }
        }
    }
}

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