java io系列24之 BufferedWriter(字符缓冲输出流)
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更多内容请参考:java io系列01之 "目录"
BufferedWriter 介绍
BufferedWriter 是缓冲字符输出流。它继承于Writer。
BufferedWriter 的作用是为其他字符输出流添加一些缓冲功能。
BufferedWriter 函数列表
// 构造函数
BufferedWriter(Writer out)
BufferedWriter(Writer out, int sz)
void close() // 关闭此流,但要先刷新它。
void flush() // 刷新该流的缓冲。
void newLine() // 写入一个行分隔符。
void write(char[] cbuf, int off, int len) // 写入字符数组的某一部分。
void write(int c) // 写入单个字符。
void write(String s, int off, int len) // 写入字符串的某一部分。
BufferedWriter 源码分析(基于jdk1.7.40)
![](https://images.cnblogs.com/OutliningIndicators/ExpandedBlockStart.gif)
1 package java.io;
2
3 public class BufferedWriter extends Writer {
4
5 // 输出流对象
6 private Writer out;
7
8 // 保存“缓冲输出流”数据的字符数组
9 private char cb[];
10
11 // nChars 是cb缓冲区中字符的总的个数
12 // nextChar 是下一个要读取的字符在cb缓冲区中的位置
13 private int nChars, nextChar;
14
15 // 默认字符缓冲区大小
16 private static int defaultCharBufferSize = 8192;
17
18 // 行分割符
19 private String lineSeparator;
20
21 // 构造函数,传入“Writer对象”,默认缓冲区大小是8k
22 public BufferedWriter(Writer out) {
23 this(out, defaultCharBufferSize);
24 }
25
26 // 构造函数,传入“Writer对象”,指定缓冲区大小是sz
27 public BufferedWriter(Writer out, int sz) {
28 super(out);
29 if (sz <= 0)
30 throw new IllegalArgumentException("Buffer size <= 0");
31 this.out = out;
32 cb = new char[sz];
33 nChars = sz;
34 nextChar = 0;
35
36 lineSeparator = java.security.AccessController.doPrivileged(
37 new sun.security.action.GetPropertyAction("line.separator"));
38 }
39
40 // 确保“BufferedWriter”是打开状态
41 private void ensureOpen() throws IOException {
42 if (out == null)
43 throw new IOException("Stream closed");
44 }
45
46 // 对缓冲区执行flush()操作,将缓冲区的数据写入到Writer中
47 void flushBuffer() throws IOException {
48 synchronized (lock) {
49 ensureOpen();
50 if (nextChar == 0)
51 return;
52 out.write(cb, 0, nextChar);
53 nextChar = 0;
54 }
55 }
56
57 // 将c写入到缓冲区中。先将c转换为char,然后将其写入到缓冲区。
58 public void write(int c) throws IOException {
59 synchronized (lock) {
60 ensureOpen();
61 // 若缓冲区满了,则清空缓冲,将缓冲数据写入到输出流中。
62 if (nextChar >= nChars)
63 flushBuffer();
64 cb[nextChar++] = (char) c;
65 }
66 }
67
68 // 返回a,b中较小的数
69 private int min(int a, int b) {
70 if (a < b) return a;
71 return b;
72 }
73
74 // 将字符数组cbuf写入到缓冲中,从cbuf的off位置开始写入,写入长度是len。
75 public void write(char cbuf[], int off, int len) throws IOException {
76 synchronized (lock) {
77 ensureOpen();
78 if ((off < 0) || (off > cbuf.length) || (len < 0) ||
79 ((off + len) > cbuf.length) || ((off + len) < 0)) {
80 throw new IndexOutOfBoundsException();
81 } else if (len == 0) {
82 return;
83 }
84
85 if (len >= nChars) {
86 /* If the request length exceeds the size of the output buffer,
87 flush the buffer and then write the data directly. In this
88 way buffered streams will cascade harmlessly. */
89 flushBuffer();
90 out.write(cbuf, off, len);
91 return;
92 }
93
94 int b = off, t = off + len;
95 while (b < t) {
96 int d = min(nChars - nextChar, t - b);
97 System.arraycopy(cbuf, b, cb, nextChar, d);
98 b += d;
99 nextChar += d;
100 if (nextChar >= nChars)
101 flushBuffer();
102 }
103 }
104 }
105
106 // 将字符串s写入到缓冲中,从s的off位置开始写入,写入长度是len。
107 public void write(String s, int off, int len) throws IOException {
108 synchronized (lock) {
109 ensureOpen();
110
111 int b = off, t = off + len;
112 while (b < t) {
113 int d = min(nChars - nextChar, t - b);
114 s.getChars(b, b + d, cb, nextChar);
115 b += d;
116 nextChar += d;
117 if (nextChar >= nChars)
118 flushBuffer();
119 }
120 }
121 }
122
123 // 将换行符写入到缓冲中
124 public void newLine() throws IOException {
125 write(lineSeparator);
126 }
127
128 // 清空缓冲区数据
129 public void flush() throws IOException {
130 synchronized (lock) {
131 flushBuffer();
132 out.flush();
133 }
134 }
135
136 public void close() throws IOException {
137 synchronized (lock) {
138 if (out == null) {
139 return;
140 }
141 try {
142 flushBuffer();
143 } finally {
144 out.close();
145 out = null;
146 cb = null;
147 }
148 }
149 }
150 }
说明: BufferedWriter的源码非常简单,这里就BufferedWriter的思想进行简单说明:BufferedWriter通过字符数组来缓冲数据,当缓冲区满或者用户调用flush()函数时,它就会将缓冲区的数据写入到输出流中。
示例代码
关于BufferedWriter中API的详细用法,参考示例代码(BufferedWriterTest.java):
1 import java.io.BufferedWriter;
2 import java.io.File;
3 import java.io.OutputStream;
4 import java.io.FileWriter;
5 import java.io.IOException;
6 import java.io.FileNotFoundException;
7 import java.lang.SecurityException;
8 import java.util.Scanner;
9
10 /**
11 * BufferedWriter 测试程序
12 *
13 * @author skywang
14 */
15 public class BufferedWriterTest {
16
17 private static final int LEN = 5;
18 // 对应英文字母“abcdefghijklmnopqrstuvwxyz”
19 //private static final char[] ArrayLetters = "abcdefghijklmnopqrstuvwxyz";
20 private static final char[] ArrayLetters = new char[] {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
21
22 public static void main(String[] args) {
23 testBufferedWriter() ;
24 }
25
26 /**
27 * BufferedWriter的API测试函数
28 */
29 private static void testBufferedWriter() {
30
31 // 创建“文件输出流”对应的BufferedWriter
32 // 它对应缓冲区的大小是16,即缓冲区的数据>=16时,会自动将缓冲区的内容写入到输出流。
33 try {
34 File file = new File("bufferwriter.txt");
35 BufferedWriter out =
36 new BufferedWriter(
37 new FileWriter(file));
38
39 // 将ArrayLetters数组的前10个字符写入到输出流中
40 out.write(ArrayLetters, 0, 10);
41 // 将“换行符\n”写入到输出流中
42 out.write('\n');
43
44 out.flush();
45 //readUserInput() ;
46
47 out.close();
48 } catch (FileNotFoundException e) {
49 e.printStackTrace();
50 } catch (SecurityException e) {
51 e.printStackTrace();
52 } catch (IOException e) {
53 e.printStackTrace();
54 }
55 }
56
57 /**
58 * 读取用户输入
59 */
60 private static void readUserInput() {
61 System.out.println("please input a text:");
62 Scanner reader=new Scanner(System.in);
63 // 等待一个输入
64 String str = reader.next();
65 System.out.printf("the input is : %s\n", str);
66 }
67 }
运行结果:
生成文件“bufferwriter.txt”,文件的内容是“abcdefghij”。