1 使用方法

　　FileInputStream即檔案輸入流，使用它從檔案中獲得位元組流，FileOutputStream即問價輸出流，使用它將位元組流寫入檔案。

1.1 方法介紹

　　FileInputStream提供的API如下：

FileInputStream(File file)         // 建立“File物件”對應的“檔案輸入流”
FileInputStream(FileDescriptor fd) // 建立“檔案描述符”對應的“檔案輸入流”
FileInputStream(String path)       // 建立“檔案(路徑為path)”對應的“檔案輸入流”
 


int      available()             // 返回“剩餘的可讀取的位元組數”或者“skip的位元組數”
void     close()                 // 關閉“檔案輸入流”
FileChannel      getChannel()    // 返回“FileChannel”
final FileDescriptor     getFD() // 返回“檔案描述符”
int      read()                  // 返回“檔案輸入流”的下一個位元組
int      read(byte[] buffer, int off, int len) // 讀取“檔案輸入流”的資料並存在到buffer，從off開始儲存，儲存長度是len。
 

long     skip(long n)    // 跳過n個位元組

　　FileOutputStream提供的API如下：

FileOutputStream(File file)                   // 建立“File物件”對應的“檔案輸入流”；預設“追加模式”是false，即“寫到輸出的流內容”不是以追加的方式新增到檔案中。
FileOutputStream(File file, boolean append)   // 建立“File物件”對應的“檔案輸入流”；指定“追加模式”。
FileOutputStream(FileDescriptor fd)           // 建立“檔案描述符”對應的“檔案輸入流”；預設“追加模式”是false，即“寫到輸出的流內容”不是以追加的方式新增到檔案中。
 

FileOutputStream(String path)                 // 建立“檔案(路徑為path)”對應的“檔案輸入流”；預設“追加模式”是false，即“寫到輸出的流內容”不是以追加的方式新增到檔案中。
FileOutputStream(String path, boolean append) // 建立“檔案(路徑為path)”對應的“檔案輸入流”；指定“追加模式”。

void                    close()      // 關閉“輸出流”
FileChannel             getChannel() // 返回“FileChannel”
final FileDescriptor    getFD()      // 返回“檔案描述符”
void                    write(byte[] buffer, int off, int len) // 將buffer寫入到“檔案輸出流”中，從buffer的off開始寫，寫入長度是len。
void                    write(int n)  // 寫入位元組n到“檔案輸出流”中

1.2 使用示例

/**
 * 在原始碼所在目錄生成一個test.txt,並寫入abcdefghijklmn123456
 */
public void testFileOutputStream() {
    try {
        byte [] content = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n'};
        //床架test.txt檔案
        File file = new File("test.txt");
        //建立檔案輸出流
        FileOutputStream outputStream = new FileOutputStream(file);
        outputStream.write(content, 0, 14);
        //PrintStream寫入方便
        PrintStream printStream = new PrintStream(outputStream);
        printStream.print("123456");
        printStream.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

/**
 * 測試檔案輸入流
 */
public void testFileInputStream() {
    try {
        //新建輸入流,檔案中的內容為abcdefghijklmn123456
        FileInputStream inputStream = new FileInputStream("test.txt");
        //讀取一個位元組a
        System.out.println("讀取一個位元組: " + inputStream.read());
        //跳過兩個位元組 b c
        inputStream.skip(2);
        //讀取三個位元組到buff中def
        byte [] buff = new byte[3];
        inputStream.read(buff, 0, 3);
        System.out.println("buff中的內容為: " + new String(buff));
        inputStream.close();
    } catch (IOException e) {
        e.printStackTrace();
    }

}

　　執行結果如下：

讀取一個位元組: 97
buff中的內容為: def

2 原始碼分析

2.1FileInputStream原始碼分析

2.1.1 構造方法

　　FileInputStream提供三個構造方法，區別是傳入的引數型別（檔案路徑，FIle物件，檔案描述符物件）。

/**
 * Creates a <code>FileInputStream</code> by
 * opening a connection to an actual file,
 * the file named by the path name <code>name</code>
 * in the file system.  A new <code>FileDescriptor</code>
 * object is created to represent this file
 * connection.
 * <p>
 * First, if there is a security
 * manager, its <code>checkRead</code> method
 * is called with the <code>name</code> argument
 * as its argument.
 * <p>
 * If the named file does not exist, is a directory rather than a regular
 * file, or for some other reason cannot be opened for reading then a
 * <code>FileNotFoundException</code> is thrown.
 *
 * @param      name   the system-dependent file name.
 * @exception  FileNotFoundException  if the file does not exist,
 *                   is a directory rather than a regular file,
 *                   or for some other reason cannot be opened for
 *                   reading.
 * @exception  SecurityException      if a security manager exists and its
 *               <code>checkRead</code> method denies read access
 *               to the file.
 * @see        java.lang.SecurityManager#checkRead(java.lang.String)
 */
public FileInputStream(String name) throws FileNotFoundException {
    this(name != null ? new File(name) : null);
}
/**
 * Creates a <code>FileInputStream</code> by
 * opening a connection to an actual file,
 * the file named by the <code>File</code>
 * object <code>file</code> in the file system.
 * A new <code>FileDescriptor</code> object
 * is created to represent this file connection.
 * <p>
 * First, if there is a security manager,
 * its <code>checkRead</code> method  is called
 * with the path represented by the <code>file</code>
 * argument as its argument.
 * <p>
 * argument as its argument.
 * <p>
 * If the named file does not exist, is a directory rather than a regular
 * file, or for some other reason cannot be opened for reading then a
 * <code>FileNotFoundException</code> is thrown.
 *
 * @param      file   the file to be opened for reading.
 * @exception  FileNotFoundException  if the file does not exist,
 *                   is a directory rather than a regular file,
 *                   or for some other reason cannot be opened for
 *                   reading.
 * @exception  SecurityException      if a security manager exists and its
 *               <code>checkRead</code> method denies read access to the file.
 * @see        java.io.File#getPath()
 * @see        java.lang.SecurityManager#checkRead(java.lang.String)
 */
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.attach(this);
    path = name;
    open(name);
}
/**
 * Creates a <code>FileInputStream</code> by using the file descriptor
 * <code>fdObj</code>, which represents an existing connection to an
 * actual file in the file system.
 * <p>
 * If there is a security manager, its <code>checkRead</code> method is
 * called with the file descriptor <code>fdObj</code> as its argument to
 * see if it's ok to read the file descriptor. If read access is denied
 * to the file descriptor a <code>SecurityException</code> is thrown.
 * <p>
 * If <code>fdObj</code> is null then a <code>NullPointerException</code>
 * is thrown.
 * <p>
 * This constructor does not throw an exception if <code>fdObj</code>
 * is {@link java.io.FileDescriptor#valid() invalid}.
 * However, if the methods are invoked on the resulting stream to attempt
 * I/O on the stream, an <code>IOException</code> is thrown.
 *
 * @param      fdObj   the file descriptor to be opened for reading.
 * @throws     SecurityException      if a security manager exists and its
 *                 <code>checkRead</code> method denies read access to the
 *                 file descriptor.
 * @see        SecurityManager#checkRead(java.io.FileDescriptor)
 */
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.
     * Register this stream with FileDescriptor tracker.
     */
    fd.attach(this);
}

2.2 FileOutputStream原始碼分析

2.1.1 構造方法

public FileOutputStream(String name) throws FileNotFoundException {
    this(name != null ? new File(name) : null, false);
}

/**
 * Creates a file output stream to write to the file with the specified
 * name.  If the second argument is <code>true</code>, then
 * bytes will be written to the end of the file rather than the beginning.
 * A new <code>FileDescriptor</code> object is created to represent this
 * file connection.
 * <p>
 * First, if there is a security manager, its <code>checkWrite</code>
 * method is called with <code>name</code> as its argument.
 * <p>
 * If the file exists but is a directory rather than a regular file, does
 * not exist but cannot be created, or cannot be opened for any other
 * reason then a <code>FileNotFoundException</code> is thrown.
 *
 * @param     name        the system-dependent file name
 * @param     append      if <code>true</code>, then bytes will be written
 *                   to the end of the file rather than the beginning
 * @exception  FileNotFoundException  if the file exists but is a directory
 *                   rather than a regular file, does not exist but cannot
 *                   be created, or cannot be opened for any other reason.
 * @exception  SecurityException  if a security manager exists and its
 *               <code>checkWrite</code> method denies write access
 *               to the file.
 * @see        java.lang.SecurityManager#checkWrite(java.lang.String)
 * @since     JDK1.1
 */
public FileOutputStream(String name, boolean append)
        throws FileNotFoundException
{
    this(name != null ? new File(name) : null, append);
}

public FileOutputStream(File file) throws FileNotFoundException {
    this(file, false);
}

/**
 * Creates a file output stream to write to the file represented by
 * the specified <code>File</code> object. If the second argument is
 * <code>true</code>, then bytes will be written to the end of the file
 * rather than the beginning. A new <code>FileDescriptor</code> object is
 * created to represent this file connection.
 * <p>
 * First, if there is a security manager, its <code>checkWrite</code>
 * method is called with the path represented by the <code>file</code>
 * argument as its argument.
 * <p>
 * If the file exists but is a directory rather than a regular file, does
 * not exist but cannot be created, or cannot be opened for any other
 * reason then a <code>FileNotFoundException</code> is thrown.
 *
 * @param      file               the file to be opened for writing.
 * @param     append      if <code>true</code>, then bytes will be written
 *                   to the end of the file rather than the beginning
 * @exception  FileNotFoundException  if the file exists but is a directory
 *                   rather than a regular file, does not exist but cannot
 *                   be created, or cannot be opened for any other reason
 * @exception  SecurityException  if a security manager exists and its
 *                   be created, or cannot be opened for any other reason
 * @exception  SecurityException  if a security manager exists and its
 *               <code>checkWrite</code> method denies write access
 *               to the file.
 * @see        java.io.File#getPath()
 * @see        java.lang.SecurityException
 * @see        java.lang.SecurityManager#checkWrite(java.lang.String)
 * @since 1.4
 */
public FileOutputStream(File file, boolean append)
        throws FileNotFoundException
{
    String name = (file != null ? file.getPath() : null);
    SecurityManager security = System.getSecurityManager();
    if (security != null) {
        security.checkWrite(name);
    }
    if (name == null) {
        throw new NullPointerException();
    }
    if (file.isInvalid()) {
        throw new FileNotFoundException("Invalid file path");
    }
    this.fd = new FileDescriptor();
    fd.attach(this);
    this.append = append;
    this.path = name;

    open(name, append);
}

/**
 * Creates a file output stream to write to the specified file
 * descriptor, which represents an existing connection to an actual
 * file in the file system.
 * <p>
 * First, if there is a security manager, its <code>checkWrite</code>
 * method is called with the file descriptor <code>fdObj</code>
 * argument as its argument.
 * <p>
 * If <code>fdObj</code> is null then a <code>NullPointerException</code>
 * is thrown.
 * <p>
 * This constructor does not throw an exception if <code>fdObj</code>
 * is {@link java.io.FileDescriptor#valid() invalid}.
 * However, if the methods are invoked on the resulting stream to attempt
 * I/O on the stream, an <code>IOException</code> is thrown.
 *
 * @param      fdObj   the file descriptor to be opened for writing
 * @exception  SecurityException  if a security manager exists and its
 *               <code>checkWrite</code> method denies
 *               write access to the file descriptor
 * @see        java.lang.SecurityManager#checkWrite(java.io.FileDescriptor)
 */
public FileOutputStream(FileDescriptor fdObj) {
    SecurityManager security = System.getSecurityManager();
    if (fdObj == null) {
        throw new NullPointerException();
    }
    if (security != null) {
        security.checkWrite(fdObj);
    }
    this.fd = fdObj;
    this.append = false;
    this.path = null;

    fd.attach(this);
}

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