java 字符缓冲流BufferedReader和BufferedWriter

本节概要:

BufferedReader

继承自Reader

构造方法:

     private static int defaultCharBufferSize = 8192;
//可以手动设置缓存去char数组的大小
     public BufferedReader(Reader in, int sz) {
        super(in);
        if (sz <= 0)
            throw new IllegalArgumentException("Buffer size <= 0");
        this.in = in;
        cb = new char[sz];
        nextChar = nChars = 0;
    }
    //创建一个char数组,大小为8192,用来缓存
    public BufferedReader(Reader in) {
        this(in, defaultCharBufferSize);
    }

工作过程:使用BufferedReader时,先从Reader对象(FileReader)中读取数据写入到char[],然后从自身的char数组中访问数据,当char数组的数据被访问完毕,就调用fill()方法重新填充char数组。关闭BufferedReader时,其构造参数的Reader也被关闭

特有方法:

String readLine() 从BufferedReader自身的char数组中读取一行文本,遇到以下字符会认为完成一行的读取,换行('\n'),回车('\r')或者回车后直接换行,返回包含该行内容的字符串,如果达到流末尾,返回null

看下BufferedReader的read方法:read()访问的BufferedReader本身的char数组，当数组的内容被访问完毕时调用调用fill()方法重新填充char数组。

   private Reader in;
   public int read(char cbuf[], int off, int len) throws IOException {
            synchronized (lock) {
            ensureOpen();
            if ((off < 0) || (off > cbuf.length) || (len < 0) ||
                ((off + len) > cbuf.length) || ((off + len) < 0)) {
                throw new IndexOutOfBoundsException();
            } else if (len == 0) {
                return 0;
            }

            int n = read1(cbuf, off, len);
            if (n <= 0) return n;
            while ((n < len) && in.ready()) {
                int n1 = read1(cbuf, off + n, len - n);
                if (n1 <= 0) break;
                n += n1;
            }
            return n; 
   private int read1(char[] cbuf, int off, int len) throws IOException {
          if (nextChar >= nChars) {
            if (len >= cb.length && markedChar <= UNMARKED && !skipLF) {
                return in.read(cbuf, off, len);
            }
            fill();
        }
        if (nextChar >= nChars) return -1;
        if (skipLF) {
            skipLF = false;
            if (cb[nextChar] == '\n') {
                nextChar++;
                if (nextChar >= nChars)
                    fill();
                if (nextChar >= nChars)
                    return -1;
            }
        }
        int n = Math.min(len, nChars - nextChar);
        System.arraycopy(cb, nextChar, cbuf, off, n);
        nextChar += n;
        return n;
    }

BufferedReader示例:

源文件word.txt内容:
好啊
你好
天天
///////////////////////////////////////////////////////////////
import java.io.*;

public class BufferedReaderTest01 {
    public static void main(String[] args){
        try (
                BufferedReader br =new BufferedReader(new FileReader("word.txt"));
                ){
            String s;
            //每次读取一行,读到末尾处为null
            while ((s=br.readLine())!=null){
                System.out.println( s);
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}
out:
好啊
你好
天天

BufferedWriter

继承自Writer

构造方法:

 private static int defaultCharBufferSize = 8192;
//创建默认8192大小的char数组,用来缓存
 public BufferedWriter(Writer out) {
        this(out, defaultCharBufferSize);
    }
//根据指定大小创建char数组,用来缓存
 public BufferedWriter(Writer out, int sz) {
        super(out);
        if (sz <= 0)
            throw new IllegalArgumentException("Buffer size <= 0");
        this.out = out;
        cb = new char[sz];
        nChars = sz;
        nextChar = 0;
    }

工作工程: BufferedWriter先将数据写入自身的char数组，当调用write方法时,将内容写到自身的char数组,再调用参数Writer对象的write方法从char数组中读取内容写入到文件中

关闭BufferedWriter时,自动关闭作为参数的Writer对象

特有方法:

newLine()换行

write方法,可以写字符串和char数组,可以指定他们的起始位置和长度,看下write的源码:

先将内容保存到自身的char数组中,再访问自身char数组,再将内容写入到文件中

private Writer out;//构造方法中传入的Writer对象
private char cb[];
//调用flushBuffer方法来写
public void write(int c) throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (nextChar >= nChars)
                flushBuffer();
            cb[nextChar++] = (char) c;
        }
    }
//调用out的write方法,out是创建BufferedWriter对象时,传进来的Writer对象
void flushBuffer() throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (nextChar == 0)
                return;
            out.write(cb, 0, nextChar);
            nextChar = 0;
        }
    }

示例:

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;

public class BufferedWriterTest01 {
    public static void main(String[] args){
        try (
                BufferedWriter bw=new BufferedWriter(new FileWriter("test\\test6.txt"));
                ){
            bw.write("我们要加油");
            bw.newLine();//换行
            char [] charArr={'我','们','热','爱','祖','国'};
            bw.write(charArr,0,4);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}
out:
test6.txt的内容为:
我们要加油
我们热爱

字符缓冲流复制文本文件:

import java.io.*;

public class FileCopy02 {
    public static void main(String[] args){
        try (
                BufferedReader br=new BufferedReader(new FileReader("word1.txt"));
                BufferedWriter bw=new BufferedWriter(new FileWriter("test\\test7.txt"));
                ){
            String s;
            //通过每次读取一行,提高效率
            while((s=br.readLine())!=null){
                bw.write(s);
                bw.newLine();
            }

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}
out:
源文件word1.txt内容:
好吗
天天
目标文件test7.txt内容:
好吗
天天

画个示意图: