Java IO：FileInputStream和FileOutputStream使用详解及源码分析

1 使用方法

　　FileInputStream即文件输入流，使用它从文件中获得字节流，FileOutputStream即问价输出流，使用它将字节流写入文件。

1.1 方法介绍

　　FileInputStream提供的API如下：

FileInputStream(File file)         // 创建“File对象”对应的“文件输入流”
FileInputStream(FileDescriptor fd) // 创建“文件描述符”对应的“文件输入流”
FileInputStream(String path)       // 创建“文件(路径为path)”对应的“文件输入流”

int      available()             // 返回“剩余的可读取的字节数”或者“skip的字节数”
void     close()                 // 关闭“文件输入流”
FileChannel      getChannel()    // 返回“FileChannel”
final FileDescriptor     getFD() // 返回“文件描述符”
int      read()                  // 返回“文件输入流”的下一个字节
int      read(byte[] buffer, int off, int len) // 读取“文件输入流”的数据并存在到buffer，从off开始存储，存储长度是len。
long     skip(long n)    // 跳过n个字节

　　FileOutputStream提供的API如下：

FileOutputStream(File file)                   // 创建“File对象”对应的“文件输入流”；默认“追加模式”是false，即“写到输出的流内容”不是以追加的方式添加到文件中。
FileOutputStream(File file, boolean append)   // 创建“File对象”对应的“文件输入流”；指定“追加模式”。
FileOutputStream(FileDescriptor fd)           // 创建“文件描述符”对应的“文件输入流”；默认“追加模式”是false，即“写到输出的流内容”不是以追加的方式添加到文件中。
FileOutputStream(String path)                 // 创建“文件(路径为path)”对应的“文件输入流”；默认“追加模式”是false，即“写到输出的流内容”不是以追加的方式添加到文件中。
FileOutputStream(String path, boolean append) // 创建“文件(路径为path)”对应的“文件输入流”；指定“追加模式”。

void                    close()      // 关闭“输出流”
FileChannel             getChannel() // 返回“FileChannel”
final FileDescriptor    getFD()      // 返回“文件描述符”
void                    write(byte[] buffer, int off, int len) // 将buffer写入到“文件输出流”中，从buffer的off开始写，写入长度是len。
void                    write(int n)  // 写入字节n到“文件输出流”中

1.2 使用示例

/**
* 在源码所在目录生成一个test.txt,并写入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文件
File file = new File("test.txt");
//创建文件输出流
FileOutputStream outputStream = new FileOutputStream(file);
outputStream.write(content, 0, 14);
//PrintStream写入方便
PrintStream printStream = new PrintStream(outputStream);
printStream.print("123456");
printStream.close();
} catch (Exception e) {
e.printStackTrace();
}
}

/**
* 测试文件输入流
*/
public void testFileInputStream() {
try {
//新建输入流,文件中的内容为abcdefghijklmn123456
FileInputStream inputStream = new FileInputStream("test.txt");
//读取一个字节a
System.out.println("读取一个字节: " + inputStream.read());
//跳过两个字节 b c
inputStream.skip(2);
//读取三个字节到buff中def
byte [] buff = new byte[3];
inputStream.read(buff, 0, 3);
System.out.println("buff中的内容为: " + new String(buff));
inputStream.close();
} catch (IOException e) {
e.printStackTrace();
}

}

　　运行结果如下：

读取一个字节: 97
buff中的内容为: def

2 源码分析

2.1FileInputStream源码分析

2.1.1 构造方法

　　FileInputStream提供三个构造方法，区别是传入的参数类型（文件路径，FIle对象，文件描述符对象）。

/**
* 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源码分析

2.1.1 构造方法

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);
}

参考：

[1] /article/4708107.html

[2] http://wangkuiwu.github.io/2012/05/07/FileInputStreamAndFileOutputStream/