java io系列19之 CharArrayWriter(字符数组输出流)

本章，我们学习CharArrayWriter。学习时，我们先对CharArrayWriter有个大致了解，然后深入了解一下它的源码，最后通过示例来掌握它的用法。

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CharArrayWriter 介绍

CharArrayReader 用于写入数据符，它继承于Writer。操作的数据是以字符为单位！

CharArrayWriter 函数列表

CharArrayWriter()
CharArrayWriter(int initialSize)

CharArrayWriter     append(CharSequence csq, int start, int end)
CharArrayWriter     append(char c)
CharArrayWriter     append(CharSequence csq)
void     close()
void     flush()
void     reset()
int     size()
char[]     toCharArray()
String     toString()
void     write(char[] buffer, int offset, int len)
void     write(int oneChar)
void     write(String str, int offset, int count)
void     writeTo(Writer out)

Writer和CharArrayWriter源码分析

Writer是CharArrayWriter的父类，我们先看看Writer的源码，然后再学CharArrayWriter的源码。

1. Writer源码分析(基于jdk1.7.40)

 package java.io;

 public abstract class Writer implements Appendable, Closeable, Flushable {

     private char[] writeBuffer;

     private final int writeBufferSize = 1024;

     protected Object lock;

     protected Writer() {
         this.lock = this;
     }

     protected Writer(Object lock) {
         if (lock == null) {
             throw new NullPointerException();
         }
         this.lock = lock;
     }

     public void write(int c) throws IOException {
         synchronized (lock) {
             if (writeBuffer == null){
                 writeBuffer = new char[writeBufferSize];
             }
             writeBuffer[0] = (char) c;
             write(writeBuffer, 0, 1);
         }
     }

     public void write(char cbuf[]) throws IOException {
         write(cbuf, 0, cbuf.length);
     }

     abstract public void write(char cbuf[], int off, int len) throws IOException;

     public void write(String str) throws IOException {
         write(str, 0, str.length());
     }

     public void write(String str, int off, int len) throws IOException {
         synchronized (lock) {
             char cbuf[];
             if (len <= writeBufferSize) {
                 if (writeBuffer == null) {
                     writeBuffer = new char[writeBufferSize];
                 }
                 cbuf = writeBuffer;
             } else {    // Don't permanently allocate very large buffers.
                 cbuf = new char[len];
             }
             str.getChars(off, (off + len), cbuf, 0);
             write(cbuf, 0, len);
         }
     }

     public Writer append(CharSequence csq) throws IOException {
         if (csq == null)
             write("null");
         else
             write(csq.toString());
         return this;
     }

     public Writer append(CharSequence csq, int start, int end) throws IOException {
         CharSequence cs = (csq == null ? "null" : csq);
         write(cs.subSequence(start, end).toString());
         return this;
     }

     public Writer append(char c) throws IOException {
         write(c);
         return this;
     }

     abstract public void flush() throws IOException;

     abstract public void close() throws IOException;
 }

2. CharArrayWriter 源码分析(基于jdk1.7.40)

 package java.io;

 import java.util.Arrays;

 public class CharArrayWriter extends Writer {
     // 字符数组缓冲
     protected char buf[];

     // 下一个字符的写入位置
     protected int count;

     // 构造函数：默认缓冲区大小是32
     public CharArrayWriter() {
         this(32);
     }

     // 构造函数：指定缓冲区大小是initialSize
     public CharArrayWriter(int initialSize) {
         if (initialSize < 0) {
             throw new IllegalArgumentException("Negative initial size: "
                                                + initialSize);
         }
         buf = new char[initialSize];
     }

     // 写入一个字符c到CharArrayWriter中
     public void write(int c) {
         synchronized (lock) {
             int newcount = count + 1;
             if (newcount > buf.length) {
                 buf = Arrays.copyOf(buf, Math.max(buf.length << 1, newcount));
             }
             buf[count] = (char)c;
             count = newcount;
         }
     }

     // 写入字符数组c到CharArrayWriter中。off是“字符数组b中的起始写入位置”，len是写入的长度
     public void write(char c[], int off, int len) {
         if ((off < 0) || (off > c.length) || (len < 0) ||
             ((off + len) > c.length) || ((off + len) < 0)) {
             throw new IndexOutOfBoundsException();
         } else if (len == 0) {
             return;
         }
         synchronized (lock) {
             int newcount = count + len;
             if (newcount > buf.length) {
                 buf = Arrays.copyOf(buf, Math.max(buf.length << 1, newcount));
             }
             System.arraycopy(c, off, buf, count, len);
             count = newcount;
         }
     }

     // 写入字符串str到CharArrayWriter中。off是“字符串的起始写入位置”，len是写入的长度
     public void write(String str, int off, int len) {
         synchronized (lock) {
             int newcount = count + len;
             if (newcount > buf.length) {
                 buf = Arrays.copyOf(buf, Math.max(buf.length << 1, newcount));
             }
             str.getChars(off, off + len, buf, count);
             count = newcount;
         }
     }

     // 将CharArrayWriter写入到“Writer对象out”中
     public void writeTo(Writer out) throws IOException {
         synchronized (lock) {
             out.write(buf, 0, count);
         }
     }

     // 将csq写入到CharArrayWriter中
     // 注意：该函数返回CharArrayWriter对象
     public CharArrayWriter append(CharSequence csq) {
         String s = (csq == null ? "null" : csq.toString());
         write(s, 0, s.length());
         return this;
     }

     // 将csq从start开始(包括)到end结束(不包括)的数据，写入到CharArrayWriter中。
     // 注意：该函数返回CharArrayWriter对象！
     public CharArrayWriter append(CharSequence csq, int start, int end) {
         String s = (csq == null ? "null" : csq).subSequence(start, end).toString();
         write(s, 0, s.length());
         return this;
     }

     // 将字符c追加到CharArrayWriter中！
     // 注意：它与write(int c)的区别。append(char c)会返回CharArrayWriter对象。
     public CharArrayWriter append(char c) {
         write(c);
         return this;
     }

     // 重置
     public void reset() {
         count = 0;
     }

     // 将CharArrayWriter的全部数据对应的char[]返回
     public char toCharArray()[] {
         synchronized (lock) {
             return Arrays.copyOf(buf, count);
         }
     }

     // 返回CharArrayWriter的大小
     public int size() {
         return count;
     }

     public String toString() {
         synchronized (lock) {
             return new String(buf, 0, count);
         }
     }

     public void flush() { }

     public void close() { }
 }

说明：
CharArrayWriter实际上是将数据写入到“字符数组”中去。
(01) 通过CharArrayWriter()创建的CharArrayWriter对应的字符数组大小是32。
(02) 通过CharArrayWriter(int size) 创建的CharArrayWriter对应的字符数组大小是size。
(03) write(int oneChar)的作用将int类型的oneChar换成char类型，然后写入到CharArrayWriter中。
(04) write(char[] buffer, int offset, int len) 是将字符数组buffer写入到输出流中，offset是从buffer中读取数据的起始偏移位置，len是读取的长度。
(05) write(String str, int offset, int count) 是将字符串str写入到输出流中，offset是从str中读取数据的起始位置，count是读取的长度。
(06) append(char c)的作用将char类型的c写入到CharArrayWriter中，然后返回CharArrayWriter对象。
注意：append(char c)与write(int c)都是将单个字符写入到CharArrayWriter中。它们的区别是，append(char c)会返回CharArrayWriter对象，但是write(int c)返回void。
(07) append(CharSequence csq, int start, int end)的作用将csq从start开始(包括)到end结束(不包括)的数据，写入到CharArrayWriter中。
注意：该函数返回CharArrayWriter对象！
(08) append(CharSequence csq)的作用将csq写入到CharArrayWriter中。
注意：该函数返回CharArrayWriter对象！
(09) writeTo(OutputStream out) 将该“字符数组输出流”的数据全部写入到“输出流out”中。

示例代码

关于CharArrayWriter中API的详细用法，参考示例代码(CharArrayWriterTest.java)：

 import java.io.CharArrayReader;
 import java.io.CharArrayWriter;
 import java.io.IOException;

 /**
  * CharArrayWriter 测试程序
  *
  * @author skywang
  */
 public class CharArrayWriterTest {

     private static final int LEN = 5;
     // 对应英文字母“abcdefghijklmnopqrstuvwxyz”
     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'};

     public static void main(String[] args) {

         tesCharArrayWriter() ;
     }

     /**
      * CharArrayWriter的API测试函数
      */
     private static void tesCharArrayWriter() {
         try {
             // 创建CharArrayWriter字符流
             CharArrayWriter caw = new CharArrayWriter();

             // 写入“A”个字符
             caw.write('A');
             // 写入字符串“BC”个字符
             caw.write("BC");
             //System.out.printf("caw=%s\n", caw);
             // 将ArrayLetters数组中从“3”开始的后5个字符(defgh)写入到caw中。
             caw.write(ArrayLetters, 3, 5);
             //System.out.printf("caw=%s\n", caw);

             // (01) 写入字符0
             // (02) 然后接着写入“123456789”
             // (03) 再接着写入ArrayLetters中第8-12个字符(ijkl)
             caw.append('0').append("123456789").append(String.valueOf(ArrayLetters), 8, 12);

             System.out.printf("caw=%s\n", caw);

             // 计算长度
             int size = caw.size();
             System.out.printf("size=%s\n", size);

             // 转换成byte[]数组
             char[] buf = caw.toCharArray();
             System.out.printf("buf=%s\n", String.valueOf(buf));

             // 将caw写入到另一个输出流中
             CharArrayWriter caw2 = new CharArrayWriter();
             caw.writeTo(caw2);
             System.out.printf("caw2=%s\n", caw2);
         } catch (IOException e) {
             e.printStackTrace();
         }
     }
 }

运行结果：

caw=ABCdefgh0123456789ijkl
size=22
buf=ABCdefgh0123456789ijkl
caw2=ABCdefgh0123456789ijkl