Java 日看一类（19）之IO包中的FileOutputStream类

该类继承自OutputStream
引入了如下包：import java.nio.channels.FileChannel;
import sun.nio.ch.FileChannelImpl;


类头注释如下：/**
* 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
*/大意如下：
文件输出流用于向FIle类或者FileDescriptor类中写入数据
文件是否可用或者是否能被创建取决于底层平台
尤其一些平台，在同一时刻一个文件只允许被一个FileOutputStream流（或者其他文件写入类）写入
在这种情况下，如果指定文件已经被打开，该类的构造方法将失败
FileOutputStream用来写入未被加工过的字节流，如图像数据
如果要写入字符流，推荐使用FileWriter

该类含有如下的成员变量：
文件描述符（系统给的private final FileDescriptor fd;判定文件是否为追加打开的（为追加打开则为trueprivate final boolean append;文件管道private FileChannel channel;文件路径（用文件描述符创建则为空private final String path;关闭锁private final Object closeLock = new Object();流关闭标志private volatile boolean closed = false;

含有如下的方法：
构造方法：传入文件路径，默认不关联public FileOutputStream(String name) throws FileNotFoundException {
this(name != null ? new File(name) : null, false);
}构造方法：传入文件路径和关联打开标示public FileOutputStream(String name, boolean append)
throws FileNotFoundException
{
this(name != null ? new File(name) : null, append);
}构造方法：默认不关联，传入File对象public FileOutputStream(File file) throws FileNotFoundException {
this(file, false);
}核心构造方法：传入File对象和关联标示public FileOutputStream(File file, boolean append)
throws FileNotFoundException
{
String name = (file != null ? file.getPath() : null);//写入文件路径名获取
SecurityManager security = System.getSecurityManager();//获得安全管理器
if (security != null) {
security.checkWrite(name);//检测文件是否能写入
}
if (name == null) {
throw new NullPointerException();
}
if (file.isInvalid()) {//路径有效性检查
throw new FileNotFoundException("Invalid file path");
}
this.fd = new FileDescriptor();//初始化
fd.attach(this);//和该类绑定
this.append = append;
this.path = name;

open(name, append);//打开文件
}构造函数：创建一个向指定文件描述符处写入数据的输出文件流，该文件描述符表示一个到文件系统中的某个实际文件的现有连接。
public FileOutputStream(FileDescriptor fdObj) {
SecurityManager security = System.getSecurityManager();
if (fdObj == null) {
throw new NullPointerException();
}
if (security != null) {
security.checkWrite(fdObj);//检查该对象能否写入
}
this.fd = fdObj;
this.append = false;
this.path = null;

fd.attach(this);
}native开启函数（应该是调用底层获得文件的打开和写入权限private native void open0(String name, boolean append)
throws FileNotFoundException;native写入函数（文件关联性在构造时就设定好了private native void write(int b, boolean append) throws IOException;写入特定字节public void write(int b) throws IOException {
write(b, append);
}native写入字节数组private native void writeBytes(byte b[], int off, int len, boolean append)
throws IOException;将传入的数组全部写入public void write(byte b[]) throws IOException {
writeBytes(b, 0, b.length, append);
}将传入数组的特定部分写入public void write(byte b[], int off, int len) throws IOException {
writeBytes(b, off, len, append);
}关闭输出流并释放该流所占用的系统资源（如果关联了FileChannel也一并关闭）public void close() throws IOException {
synchronized (closeLock) {
if (closed) {
return;
}
closed = true;//修改标识符
}

if (channel != null) {//关闭文件频道
channel.close();
}

fd.closeAll(new Closeable() {//关闭文件描述符所关联的文件，并注销描述符（非标准流
public void close() throws IOException {
close0();
}
});
}获得文件描述符（存在public final FileDescriptor getFD() throws IOException {
if (fd != null) {
return fd;
}
throw new IOException();
}返回与该文件输出流连接的FileChannel对象public FileChannel getChannel() {
synchronized (this) {
if (channel == null) {
channel = FileChannelImpl.open(fd, path, false, true, append, this);
}
return channel;
}
}关闭所有与该文件的链接并确定close方法被调用protected void finalize() throws IOException {
if (fd != null) {//文件描述符还存在（未被注销
if (fd == FileDescriptor.out || fd == FileDescriptor.err) {//如果是标准输出和错误流，则清空缓冲区
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();
}
}
}关闭并注销该对象private native void close0() throws IOException;初始化JNIprivate static native void initIDs();

static {
initIDs();
}

该类也是常用类型，希望大家可以尽量熟悉该类用法和内容