概述
FileChannel是用來讀，寫，映射, 操作文件的channel(概念參見我的另一篇Java NIO概述).
A channel for reading, writing, mapping, and manipulating a file.
獲取FileChannel對象有三種方式:
FileInputStream.getChannel();
FileOutputStream.getChannel();
RandomAccessFile.getChannel();FileChannel特點
- 不能註冊到selector
- 只能阻塞的運行(就是channel的read方法是阻塞的)
- FileChannel對象是線程安全的
上代碼
public static void testFileNio(){
FileInputStream is = null;
FileChannel inChannel = null;
FileOutputStream fos = null;
FileChannel outChannel = null;
try {
is = new FileInputStream("f:/text.txt");
inChannel = is.getChannel();
System.out.println("file size --->"+inChannel.size());
ByteBuffer buffer = ByteBuffer.allocate(2);
System.out.println("buffer init position --->"+buffer.position()+"---- buffer init remaining --->"+buffer.remaining());
//這裏標記了後面纔可以調用buffer.reset(), 而且只能調用一次,
//不然會拋出java.nio.InvalidMarkException
//buffer.mark();
int flag = 0;
int i = 1;
while(flag != -1){
System.out.println("this is "+i+" time");
//返回讀取的字節數, 當讀到末尾時, 可能返回0 或者 -1
flag = inChannel.read(buffer);
if(flag == -1 || flag == 0){
break;
}
System.out.println("buffer after read , position-->"+buffer.position()+"---remaining-->"+buffer.remaining());
//寫數據
fos = new FileOutputStream(new File("f:/text2.txt"),true);
outChannel = fos.getChannel();
if(buffer.remaining() > "this is the end".getBytes().length ){
buffer.put("this is the end".getBytes());
}
System.out.println("buffer after append--position-->"+buffer.position()+"---remaining-->"+buffer.remaining()+"--- flag --->"+flag);
//開始寫, remaining也變成了當前的position,然後將position置爲0
buffer.flip();
System.out.println("buffer after flip position-->"+buffer.position()+"---remaining-->"+buffer.remaining());
while(buffer.hasRemaining()){
outChannel.write(buffer);
}
System.out.println("buffer after write, position-->"+buffer.position()+"---remaining-->"+buffer.remaining());
//和buffer.reset()的區別, buffer.reset()是回到buffer.mark()的位置
//此處不能調用buffer.flip(),;
//clear方法會將position置0, limit置爲capacity, 也就是remaining是capacity的值
buffer.clear();
System.out.println("buffer clear position --->"+buffer.position()+"---- buffer clear remaining --->"+buffer.remaining());
i++;
}
} catch (IOException e) {
e.printStackTrace();
}finally{
//本來想省略以下代碼的, 但是還是忍不住嘮叨幾句
//1.空指針判斷, 這個應該沒啥
//2. 爲什麼不是所有的close操作都寫在一個catch塊了,而是寫的這麼冗長
// 原因是如果一個close操作拋出異常, 其他close操作依然可以執行; 如果寫在一個catch塊，第一個close拋出異常的話, 其他close操作都將無法進行
if(is != null){
try {
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if(inChannel != null){
try{
inChannel.close();
}catch (IOException e){
e.printStackTrace();
}
}
if(fos != null){
try{
fos.close();
}catch (IOException e){
e.printStackTrace();
}
}
if(outChannel != null){
try{
outChannel.close();
}catch(IOException e){
e.printStackTrace();
}
}
}
}
//輸出結果如下
file size --->10
buffer init position --->0---- buffer init remaining --->2
this is 1 time
buffer after read , position-->2---remaining-->0
buffer after append--position-->2---remaining-->0--- flag --->2
buffer after flip position-->0---remaining-->2
buffer after write, position-->2---remaining-->0
buffer clear position --->0---- buffer clear remaining --->2
this is 2 time
buffer after read , position-->2---remaining-->0
buffer after append--position-->2---remaining-->0--- flag --->2
buffer after flip position-->0---remaining-->2
buffer after write, position-->2---remaining-->0
buffer clear position --->0---- buffer clear remaining --->2
this is 3 time
buffer after read , position-->2---remaining-->0
buffer after append--position-->2---remaining-->0--- flag --->2
buffer after flip position-->0---remaining-->2
buffer after write, position-->2---remaining-->0
buffer clear position --->0---- buffer clear remaining --->2
this is 4 time
buffer after read , position-->2---remaining-->0
buffer after append--position-->2---remaining-->0--- flag --->2
buffer after flip position-->0---remaining-->2
buffer after write, position-->2---remaining-->0
buffer clear position --->0---- buffer clear remaining --->2
this is 5 time
buffer after read , position-->2---remaining-->0
buffer after append--position-->2---remaining-->0--- flag --->2
buffer after flip position-->0---remaining-->2
buffer after write, position-->2---remaining-->0
buffer clear position --->0---- buffer clear remaining --->2
this is 6 time問題
既然FileChannel只能阻塞的運行, 不能註冊到selector，那麼與FileInputStream，FileOutputStream, RandomAccessFile有何區別呢？
答案： 其實在Java SE API文檔裏已經給出答案了,如下:
In addition to the familiar read, write, and close operations of byte channels, this class defines the following file-specific
operations:Bytes may be read or written at an absolute position in a file in a
way that does not affect the channel’s current position.A region of a file may be mapped directly into memory; for large files
this is often much more efficient than invoking the usual read or
write methods.Updates made to a file may be forced out to the underlying storage
device, ensuring that data are not lost in the event of a system
crash.Bytes can be transferred from a file to some other channel, and vice
versa, in a way that can be optimized by many operating systems into a
very fast transfer directly to or from the filesystem cache.A region of a file may be locked against access by other programs.