参考源码
/* * Copyright (c) 1994, 2013, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.io; import java.nio.channels.FileChannel; import sun.nio.ch.FileChannelImpl; /** * A file output stream is an output stream for writing data to a * <code>File</code> or to a <code>FileDescriptor</code>. Whether or not * a file is available or may be created depends upon the underlying * platform. Some platforms, in particular, allow a file to be opened * for writing by only one <tt>FileOutputStream</tt> (or other * file-writing object) at a time. In such situations the constructors in * this class will fail if the file involved is already open. * * <p><code>FileOutputStream</code> is meant for writing streams of raw bytes * such as image data. For writing streams of characters, consider using * <code>FileWriter</code>. * * @author Arthur van Hoff * @see java.io.File * @see java.io.FileDescriptor * @see java.io.FileInputStream * @see java.nio.file.Files#newOutputStream * @since JDK1.0 */ public class FileOutputStream extends OutputStream { /** * The system dependent file descriptor. */ private final FileDescriptor fd; /** * True if the file is opened for append. */ private final boolean append; /** * The associated channel, initialized lazily. */ private FileChannel channel; /** * The path of the referenced file * (null if the stream is created with a file descriptor) */ private final String path; private final Object closeLock = new Object(); private volatile boolean closed = false; /** * Creates a file output stream to write to the file with the * specified name. 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 filename * @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) */ 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); } /** * Creates a file output stream to write to the file represented by * the specified <code>File</code> object. 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. * @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.io.File#getPath() * @see java.lang.SecurityException * @see java.lang.SecurityManager#checkWrite(java.lang.String) */ 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 * <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); } /** * Opens a file, with the specified name, for overwriting or appending. * @param name name of file to be opened * @param append whether the file is to be opened in append mode */ private native void open(String name, boolean append) throws FileNotFoundException; /** * Writes the specified byte to this file output stream. * * @param b the byte to be written. * @param append {@code true} if the write operation first * advances the position to the end of file */ private native void write(int b, boolean append) throws IOException; /** * Writes the specified byte to this file output stream. Implements * the <code>write</code> method of <code>OutputStream</code>. * * @param b the byte to be written. * @exception IOException if an I/O error occurs. */ public void write(int b) throws IOException { write(b, append); } /** * Writes a sub array as a sequence of bytes. * @param b the data to be written * @param off the start offset in the data * @param len the number of bytes that are written * @param append {@code true} to first advance the position to the * end of file * @exception IOException If an I/O error has occurred. */ private native void writeBytes(byte b[], int off, int len, boolean append) throws IOException; /** * Writes <code>b.length</code> bytes from the specified byte array * to this file output stream. * * @param b the data. * @exception IOException if an I/O error occurs. */ public void write(byte b[]) throws IOException { writeBytes(b, 0, b.length, append); } /** * Writes <code>len</code> bytes from the specified byte array * starting at offset <code>off</code> to this file output stream. * * @param b the data. * @param off the start offset in the data. * @param len the number of bytes to write. * @exception IOException if an I/O error occurs. */ public void write(byte b[], int off, int len) throws IOException { writeBytes(b, off, len, append); } /** * Closes this file output stream and releases any system resources * associated with this stream. This file output stream may no longer * be used for writing bytes. * * <p> If this stream has an associated channel then the channel is closed * as well. * * @exception IOException if an I/O error occurs. * * @revised 1.4 * @spec JSR-51 */ public void close() throws IOException { synchronized (closeLock) { if (closed) { return; } closed = true; } if (channel != null) { channel.close(); } fd.closeAll(new Closeable() { public void close() throws IOException { close0(); } }); } /** * Returns the file descriptor associated with this stream. * * @return the <code>FileDescriptor</code> object that represents * the connection to the file in the file system being used * by this <code>FileOutputStream</code> object. * * @exception IOException if an I/O error occurs. * @see java.io.FileDescriptor */ public final FileDescriptor getFD() throws IOException { if (fd != null) { return fd; } throw new IOException(); } /** * Returns the unique {@link java.nio.channels.FileChannel FileChannel} * object associated with this file output stream. * * <p> The initial {@link java.nio.channels.FileChannel#position() * position} of the returned channel will be equal to the * number of bytes written to the file so far unless this stream is in * append mode, in which case it will be equal to the size of the file. * Writing bytes to this stream will increment the channel\'s position * accordingly. Changing the channel\'s position, either explicitly or by * writing, will change this stream\'s file position. * * @return the file channel associated with this file output stream * * @since 1.4 * @spec JSR-51 */ public FileChannel getChannel() { synchronized (this) { if (channel == null) { channel = FileChannelImpl.open(fd, path, false, true, append, this); } return channel; } } /** * Cleans up the connection to the file, and ensures that the * <code>close</code> method of this file output stream is * called when there are no more references to this stream. * * @exception IOException if an I/O error occurs. * @see java.io.FileInputStream#close() */ protected void finalize() throws IOException { if (fd != null) { if (fd == FileDescriptor.out || fd == FileDescriptor.err) { flush(); } else { /* if fd is shared, the references in FileDescriptor * will ensure that finalizer is only called when * safe to do so. All references using the fd have * become unreachable. We can call close() */ close(); } } } private native void close0() throws IOException; private static native void initIDs(); static { initIDs(); } }
构造方法
FileOutputStream(File file, boolean append)
public FileOutputStream(File file) throws FileNotFoundException {
this(file, false);
}
我们在写入文件的时候常用 new FileOutputStream(new File("filepath")); 这种构造方法，该构造方法默认的append参数值为false，即：默认相同文件覆盖写入，当需要追加写入时，第二个参数设置成true即可。