更多 Java IO & NIO方面的文章，请参见文集《Java IO & NIO》
FileInputStream & FileOutputStream
读写格式：按字节读写，每次读写一个字节，或者读写一个字节数组。
public static void main(String[] args) throws Exception {
InputStream fin = new FileInputStream("a.txt");
OutputStream fout = new FileOutputStream("b.txt");
System.out.println("How many bytes? = " + fin.available());
int c;
while((c = fin.read()) != -1) {
System.out.print((char) c);
fout.write(c);
}
}
输出：
How many bytes? = 12
Hello
World
使用缓冲流 BufferedInputStream & BufferedOutputStream
读写格式：按字节读写，每次读写一个字节，或者读写一个字节数组。
优势：使用缓冲流，更节省时间，减少访问磁盘的数目
缓冲区默认大小为 8M：private static int DEFAULT_BUFFER_SIZE = 8192;
public static void main(String[] args) throws Exception {
InputStream fin = new FileInputStream("a.txt");
BufferedInputStream bin = new BufferedInputStream(fin);
OutputStream fout = new FileOutputStream("b.txt");
BufferedOutputStream bout = new BufferedOutputStream(fout);
System.out.println("How many bytes? = " + bin.available());
// Read and Write by byte
int c;
while ((c = bin.read()) != -1) {
System.out.print((char) c);
bout.write(c);
}
// Read and Write by byte array
byte[] buf = new byte[12];
while (bin.read(buf) != -1) {
for(byte b: buf)
System.out.print((char) b);
bout.write(buf);
}
}
输出：
How many bytes? = 12
Hello
World
FileInputStream VS BufferredInputStream
对比以上两个例子：
一个使用 FileInputStream 来操作文件
另外一个使用 BufferredInputStream 来封装 FileInputStream，然后操作文件。效率更高！！！
原因分析：
FileInputStream 的 read 方法实现如下：
可以看出，实际上调用了一个本地方法，每次读取一个字节，都需要通过 OS 来访问磁盘。
/**
* Reads a byte of data from this input stream. This method blocks
* if no input is yet available.
*
* @return the next byte of data, or -1 if the end of the
* file is reached.
* @exception IOException if an I/O error occurs.
*/
public int read() throws IOException {
return read0();
}
private native int read0() throws IOException;
BufferedInputStream 的 read 方法实现如下：
可以看出，BufferedInputStream 有一个缓冲区，每次从磁盘中读取 8M 的字节，存储到内存缓冲区中，调用 read() 时实际上是读取缓冲区，无需访问磁盘。
public synchronized int read() throws IOException {
if (pos >= count) {
fill();
if (pos >= count)
return -1;
}
return getBufIfOpen()[pos++] & 0xff;
}
读取具体数据类型 DateInputStream & DateOutputStream
读写格式：按字节读写，按照具体的数据类型读写特定数目的字节：
- readChar, writeChar：每次读写 2 个字节
public final char readChar() throws IOException {
int ch1 = in.read();
int ch2 = in.read();
if ((ch1 | ch2) < 0)
throw new EOFException();
return (char)((ch1 << 8) + (ch2 << 0));
}
public final void writeChar(int v) throws IOException {
out.write((v >>> 8) & 0xFF);
out.write((v >>> 0) & 0xFF);
incCount(2);
}
- readInt, writeInt：每次读写 4 个字节
public final int readInt() throws IOException {
int ch1 = in.read();
int ch2 = in.read();
int ch3 = in.read();
int ch4 = in.read();
if ((ch1 | ch2 | ch3 | ch4) < 0)
throw new EOFException();
return ((ch1 << 24) + (ch2 << 16) + (ch3 << 8) + (ch4 << 0));
}
public final void writeInt(int v) throws IOException {
out.write((v >>> 24) & 0xFF);
out.write((v >>> 16) & 0xFF);
out.write((v >>> 8) & 0xFF);
out.write((v >>> 0) & 0xFF);
incCount(4);
}
- readDouble, writeDouble：每次读写 8 个字节
- readLong, writeLong：每次读写 8 个字节
- ...
DateInputStream & DateOutputStream 的使用：
public static void main(String[] args) throws Exception {
OutputStream fout = new FileOutputStream("chars.txt");
DataOutputStream dout = new DataOutputStream(fout);
char chars[] = {'A', 'B', 'C', 'D'};
for (int c : chars) {
dout.writeChar(c);
}
dout.flush();
InputStream fin = new FileInputStream("chars.txt");
DataInputStream din = new DataInputStream(fin);
char c;
while (din.available() > 0) {
c = din.readChar();
System.out.print(c);
}
}
输出：
ABCD
注意事项：
如下代码会将 1 按照二进制形式写入文件中，占据 4 bytes。
因此用记事本打开该文本文件，其实无法看到数字 1。
int i = 1;
dout.writeInt(i);
读取对象 ObjectInputStream & ObjectOutputStream
读写格式：按字节读写，读写的对象需要实现 Serializable 接口。
ObjectInputStream & ObjectOutputStream 的使用：
public class Stream_Object_Test {
public static void main(String[] args) throws Exception {
OutputStream fout = new FileOutputStream("people.txt");
ObjectOutputStream oout = new ObjectOutputStream(fout);
People p = new People("Tom", 20);
oout.writeObject(p);
InputStream fin = new FileInputStream("people.txt");
ObjectInputStream oin = new ObjectInputStream(fin);
People new_p = (People) oin.readObject();
System.out.println(new_p.getName() + " " + new_p.getAge());
}
}
class People implements Serializable {
private String name;
private int age;
People(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}