Java文件操作类效率对比

前言

众所周知，Java中有多种针对文件的操作类，以面向字节流和字符流可分为两大类，这里以写入为例：

面向字节流的：FileOutputStream 和 BufferedOutputStream

面向字符流的：FileWriter 和 BufferedWriter

近年来发展出New I/O ，也叫NIO，里面又包装了两个类：NewOutputStream 和 NewBufferedWriter

现在，我们建立测试程序，比较这些类写入文件的性能。

机器配置

• Processor Name: Intel Core i7
• Processor Speed: 2.2 GHz
• Number of Processors: 1
• Total Number of Cores: 4
• L2 Cache (per Core): 256 KB
• L3 Cache: 6 MB
• Memory: 16 GB

测试程序

纵向比较：几种文件操作类向文件中写入相同行数的内容（每行内容均为“写入文件Data\n”），比较其耗费时间

横向比较：对于同一个文件操作类，比较写入不同行数内容情况下所耗费时间；本文以2的次方指数级增长行数

 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.*;
 import java.nio.file.Files;
 import java.nio.file.Paths;

 public class testFileIO {

     public static void testDriver () throws IOException {
         int maxlineNum = 100000001;//写入文件的最大行数
         int startlineNum = 1;//写入文件的行数
         int Multiplying = 2;//行数增长倍率

         long begin = 0L;
         long end = 0L;

         //将时间统计写入文件Result.txt中
         FileWriter fileWriter = new FileWriter("./Result.txt", true);
         BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);

         System.out.println("Test FileOutputStream begin.");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testFileOutputStream(lineNum);
             end = System.currentTimeMillis();
             long timeElapse_FileOutputStream = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_FileOutputStream)+"\t");
         }
         System.out.println("Test FileOutputStream end.\n");

         System.out.println("Test BufferedOutputStream begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testBufferedOutputStream(lineNum);
             end = System.currentTimeMillis();
             long timeElapse_BufferedOutputStream = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_BufferedOutputStream)+"\t");
         }
         System.out.println("Test BufferedOutputStream end.\n");

         System.out.println("Test FileWriter begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testFileWriter(lineNum);
             end = System.currentTimeMillis();
             long timeElapse_FileWriter = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_FileWriter)+"\t");
         }
         System.out.println("Test FileWriter end.\n");

         System.out.println("Test BufferedWriter begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testBufferedWriter(lineNum);
             end = System.currentTimeMillis();
             long timeElapse_BufferedWriter = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_BufferedWriter)+"\t");
         }
         System.out.println("Test BufferedWriter end.\n");

         System.out.println("Test NewOutputStream begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testNewOutputStream(lineNum);
             end = System.currentTimeMillis();
             long timeElapse_NewOutputStream = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_NewOutputStream)+"\t");
         }
         System.out.println("Test NewOutputStream end.\n");

         System.out.println("Test NewBufferedWriter begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testNewBufferedWriter(lineNum);
             end = System.currentTimeMillis();
             long timeElapse_NewBufferedWriter = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_NewBufferedWriter)+"\t");
         }
         System.out.println("Test NewOutputStream end.\n");

         bufferedWriter.close();
     }

     /************************** I/O *****************************/
     //面向字节
     public static void testFileOutputStream (int lineNum) throws IOException {
         FileOutputStream fileOutputStream = new FileOutputStream(new File("./testFileOutputStream.txt"));
         while (--lineNum > 0) {
             fileOutputStream.write("写入文件Data\n".getBytes());
         }
         fileOutputStream.close();
     }

     public static void testBufferedOutputStream (int lineNum) throws IOException {
         FileOutputStream fileOutputStream = new FileOutputStream(new File("./testBufferedOutputStream.txt"));
         BufferedOutputStream bufferedOutputStream = new BufferedOutputStream(fileOutputStream);
         while (--lineNum > 0) {
             bufferedOutputStream.write("写入文件Data\n".getBytes());
         }
         bufferedOutputStream.close();
     }

     //面向字符
     public static void testFileWriter (int lineNum) throws IOException {
         FileWriter fileWriter = new FileWriter("./testFileWriter.txt");
         while (--lineNum > 0) {
             fileWriter.write("写入文件Data\n");
         }
         fileWriter.close();
     }

     public static void testBufferedWriter (int lineNum) throws IOException {
         FileWriter fileWriter = new FileWriter("./testBufferedWriter.txt");
         BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
         while (--lineNum > 0) {
             bufferedWriter.write("写入文件Data\n");
         }
         bufferedWriter.close();
     }

     /************************** NIO ****************************/
     public static void testNewOutputStream (int lineNum) throws IOException {
         OutputStream outputStream = Files.newOutputStream(Paths.get("./testNewOutputStream.txt"));
         while (--lineNum > 0) {
             outputStream.write("写入文件Data\n".getBytes());
         }
         outputStream.close();
     }

     public static void testNewBufferedWriter (int lineNum) throws IOException {
         BufferedWriter newBufferedReader = Files.newBufferedWriter(Paths.get("./testNewBufferedWriter.txt"));
         while (--lineNum > 0) {
             newBufferedReader.write("写入文件Data\n");
         }
         newBufferedReader.close();
     }

     public static void main (String[] args) throws IOException {
         //多次测试时可清空result.txt文件
         FileWriter fileWriter = new FileWriter("./Result.txt");
         testDriver();
     }
 }

测试结果

从上图可以看出，写入行数超过20W以上时，FileOutputStream和NewOutputStream耗费时间远远超出其他4个类。为了清晰，让我们放大其他4个类的图：

可以看出，这4个类中，BufferWriter和NewBufferedWriter所耗费时间更少，但总体差别不是很大。

让我们再来看看，写入26W行数据以下时的情况：

可以看出，在数据量较小的情况下，这4个类所耗费时间的差异并不是很大，在更小的数据量下，它们的效率几乎没有差别。

后记

从以上分析可知（注意横坐标写入行数是指数级增加的），各个类的时间复杂度大致为O(k)，其中不同的类的k不同，导致了最终巨大的差异。

这里只给出了测试结果，并未很深入地分析其底层实现原理，欢迎评论区留言。

另外，我没有在其他机器测试，有兴趣的小伙伴可以将自己的测试结果发出来，共同进步^_^

附件

本次测试数据结果(若看不清，可以将浏览器字体放大，或下载到本地看)

~~~~~~~~~~~~~~~~~~~~~分割线~~~~~~~~~~~~~~~~~~~~~~~

评论区小伙伴“ andorxor”提出：

XXXOutputStream是用来写二进制的，你把字符串转换成字节数组再写自然就慢了，主要慢在转换的过程。

因此，将程序修改，提前把字符和字节内容都准备好，再次验证。新程序如下：

 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.*;
 import java.nio.file.Files;
 import java.nio.file.Paths;

 public class testFileIO {

     public static void testDriver () throws IOException {
         int maxlineNum = 100000001;//写入文件的最大行数
         int startlineNum = 1;//写入文件的行数
         int Multiplying = 2;//行数增长倍率

         String contentChars = "写入文件Data\n";//每行的内容（字符流）
         byte[] contentBytes = "写入文件Data\n".getBytes();//每行的内容（字节流）

         long begin = 0L;
         long end = 0L;

         //将时间统计写入文件Result.txt中
         FileWriter fileWriter = new FileWriter("./Result.txt", true);
         BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);

         System.out.println("Test FileOutputStream begin.");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testFileOutputStream(lineNum,contentBytes);
             end = System.currentTimeMillis();
             long timeElapse_FileOutputStream = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_FileOutputStream)+"\t");
         }
         System.out.println("Test FileOutputStream end.\n");

         System.out.println("Test BufferedOutputStream begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testBufferedOutputStream(lineNum,contentBytes);
             end = System.currentTimeMillis();
             long timeElapse_BufferedOutputStream = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_BufferedOutputStream)+"\t");
         }
         System.out.println("Test BufferedOutputStream end.\n");

         System.out.println("Test FileWriter begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testFileWriter(lineNum,contentChars);
             end = System.currentTimeMillis();
             long timeElapse_FileWriter = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_FileWriter)+"\t");
         }
         System.out.println("Test FileWriter end.\n");

         System.out.println("Test BufferedWriter begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testBufferedWriter(lineNum,contentChars);
             end = System.currentTimeMillis();
             long timeElapse_BufferedWriter = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_BufferedWriter)+"\t");
         }
         System.out.println("Test BufferedWriter end.\n");

         System.out.println("Test NewOutputStream begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testNewOutputStream(lineNum,contentBytes);
             end = System.currentTimeMillis();
             long timeElapse_NewOutputStream = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_NewOutputStream)+"\t");
         }
         System.out.println("Test NewOutputStream end.\n");

         System.out.println("Test NewBufferedWriter begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testNewBufferedWriter(lineNum,contentChars);
             end = System.currentTimeMillis();
             long timeElapse_NewBufferedWriter = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_NewBufferedWriter)+"\t");
         }
         System.out.println("Test NewOutputStream end.\n");

         bufferedWriter.close();
     }

     /************************** I/O *****************************/
     //面向字节
     public static void testFileOutputStream (int lineNum, byte[] content) throws IOException {
         FileOutputStream fileOutputStream = new FileOutputStream(new File("./testFileOutputStream.txt"));
         while (--lineNum > 0) {
             fileOutputStream.write(content);
         }
         fileOutputStream.close();
     }

     public static void testBufferedOutputStream (int lineNum, byte[] content) throws IOException {
         FileOutputStream fileOutputStream = new FileOutputStream(new File("./testBufferedOutputStream.txt"));
         BufferedOutputStream bufferedOutputStream = new BufferedOutputStream(fileOutputStream);
         while (--lineNum > 0) {
             bufferedOutputStream.write(content);
         }
         bufferedOutputStream.close();
     }

     //面向字符
     public static void testFileWriter (int lineNum, String content) throws IOException {
         FileWriter fileWriter = new FileWriter("./testFileWriter.txt");
         while (--lineNum > 0) {
             fileWriter.write(content);
         }
         fileWriter.close();
     }

     public static void testBufferedWriter (int lineNum, String content) throws IOException {
         FileWriter fileWriter = new FileWriter("./testBufferedWriter.txt");
         BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
         while (--lineNum > 0) {
             bufferedWriter.write(content);
         }
         bufferedWriter.close();
     }

     /************************** NIO ****************************/
     public static void testNewOutputStream (int lineNum, byte[] content) throws IOException {
         OutputStream outputStream = Files.newOutputStream(Paths.get("./testNewOutputStream.txt"));
         while (--lineNum > 0) {
             outputStream.write(content);
         }
         outputStream.close();
     }

     public static void testNewBufferedWriter (int lineNum,String content) throws IOException {
         BufferedWriter newBufferedReader = Files.newBufferedWriter(Paths.get("./testNewBufferedWriter.txt"));
         while (--lineNum > 0) {
             newBufferedReader.write(content);
         }
         newBufferedReader.close();
     }

     public static void main (String[] args) throws IOException {
         //多次测试时可清空result.txt文件
         FileWriter fileWriter = new FileWriter("./Result.txt");
         testDriver();
     }
 }

结果为：

可以看出和前面的案例几乎没有差异（图就不画了）。

所以XXXOutputStream效率低的原因并不是字符串转换成字节数组，而是其本身的实现方式所致。

~~~~~~~~~~~~~~~~~~~~~分割线：底层实现原理浅谈~~~~~~~~~~~~~~~~~~~~~~~

其实，计算机中都是针对字节操作的（即字符都要经过编码转换为字节），那么问题来了，FileOutputStream为什么比FileWriter（FileWriter内部还有FileOutputStream转换操作，具体看源码）还要慢呢？且慢，让我们把写入文件的数据改一下：

 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.*;
 import java.nio.file.Files;
 import java.nio.file.Paths;

 public class testFileIO {

     public static void testDriver () throws IOException {
         int maxlineNum = 500001;//写入文件的最大行数
         int startlineNum = 1;//写入文件的行数
         int Multiplying = 2;//行数增长倍率

         String baseContent = "背景\n" +
             "考虑以下场景：\n" +
             "\n" +
             "InfoTable（信息表）:\n" +
             "\n" +
             "Name\tGender\tAge\tScore\n" +
             "张三\t男\t21\t90\n" +
             "李四\t女\t20\t87\n" +
             "王五\t男\t22\t92\n" +
             "赵六\t女\t19\t94\n" +
             "孙七\t女\t23\t88\n" +
             "周八\t男\t20\t91\n" +
             "StatusTable(状态表，指是否有在考试之前复习)：\n" +
             "\n" +
             "Name\thasReview\n" +
             "张三\t是\n" +
             "李四\t否\n" +
             "王五\t是\n" +
             "赵六\t是\n" +
             "孙七\t否\n" +
             "周八\t是\n" +
             "现在，我想知道所有复习过的学生的成绩，可以利用mysql中的子查询来实现：\n" +
             "\n" +
             "SELECT Score \n" +
             "FROM InfoTable \n" +
             "WHERE Name in (SELECT Name \n" +
             "               FROM StatusTable \n" +
             "               WHERE hasReview = '是');\n" +
             "这种方式非常方便，我们只要把查询条件写出来，剩下的操作都由mysql来处理。而在实际场景中，为了减少底层耦合，我们一般不通过mysql中的子查询方式联表查询，而是先执行子查询得到结果集，再以结果集作为条件执行外层查询。通常情况下，子查询和外层查询由上层的不同服务执行，这样就在一定程度上达到了底层数据库解耦的目的。注意这种实现方式将mysql内部的一部分复杂操作抛给了我们。这时，Mybatis中的foreach标签就有了用武之地。\n" +
             "\n" +
             "Mybatis 中foreach标签的用法\n" +
             "还以刚才的例子来说，先执行子查询\n" +
             "\n" +
             "SELECT Name FROM StatusTable WHERE hasReview = '是'\n" +
             "再执行外层查询，就是\n" +
             "\n" +
             "SELECT Score \n" +
             "FROM InfoTable \n" +
             "WHERE Name in ('张三' , '王五', '赵六', '周八');\n" +
             "也就是一个批量查询操作，将其抽象一下（假设有三个条件）：\n" +
             "\n" +
             "SELECT * \n" +
             "FROM <tableName> \n" +
             "WHERE <ColumnName> IN (<case1>,<case2>,<case3>)\n" +
             "实际情况中，case可能远不止3个，这时可以在XXXMapper.xml文件中利用Mybatis中的foreach编写sql语句：\n" +
             "\n" +
             "SELECT * \n" +
             "FROM <tableName> \n" +
             "WHERE <ColumnName> IN \n" +
             "<foreach collection=\"list\" index=\"index\" item=\"item\" open=\"(\" separator=\",\" close=\")\">\n" +
             "    #{item}\n" +
             "</foreach>\n" +
             "就可以实现相同的效果了。\n" +
             "\n" +
             "那么问题来了，foreach标签中各种参数是什么含义呢？\n" +
             "\n" +
             "collection\n" +
             "如果传入的是单参数且参数类型是一个List的时候，collection属性值为list\n" +
             "如果传入的是单参数且参数类型是一个array数组的时候，collection的属性值为array\n" +
             "如果传入的参数是多个的时候，我们就需要把它们封装成一个Map了，当然单参数也可以封装成map，实际上如果你在传入参数的时候，在breast里面也是会把它封装成一个Map的，map的key就是参数名，所以这个时候collection属性值就是传入的List或array对象在自己封装的map里面的key\n" +
             "index 集合迭代位置\n" +
             "item 集合中的每一个元素别名\n" +
             "open 开始符号，例如这里的(，就对应于IN (<case1>,<case2>,<case3>)中IN后面的第一个(\n" +
             "separator 分隔符，例如这里的,，就对应于IN (<case1>,<case2>,<case3>)中的,\n" +
             "close 结束符号，例如这里的)，就对应于IN (<case1>,<case2>,<case3>)中<case3>后面的)\n" +
             "参考\n";

         String contentChars = baseContent;//每行的内容（字符流）
         byte[] contentBytes = baseContent.getBytes();//每行的内容（字节流）

         long begin = 0L;
         long end = 0L;

         //将时间统计写入文件Result.txt中
         FileWriter fileWriter = new FileWriter("./Result.txt", true);
         BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);

         System.out.println("Test FileOutputStream begin.");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testFileOutputStream(lineNum,contentBytes);
             end = System.currentTimeMillis();
             long timeElapse_FileOutputStream = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_FileOutputStream)+"\t");
         }
         System.out.println("Test FileOutputStream end.\n");

         System.out.println("Test BufferedOutputStream begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testBufferedOutputStream(lineNum,contentBytes);
             end = System.currentTimeMillis();
             long timeElapse_BufferedOutputStream = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_BufferedOutputStream)+"\t");
         }
         System.out.println("Test BufferedOutputStream end.\n");

         System.out.println("Test FileWriter begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testFileWriter(lineNum,contentChars);
             end = System.currentTimeMillis();
             long timeElapse_FileWriter = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_FileWriter)+"\t");
         }
         System.out.println("Test FileWriter end.\n");

         System.out.println("Test BufferedWriter begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testBufferedWriter(lineNum,contentChars);
             end = System.currentTimeMillis();
             long timeElapse_BufferedWriter = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_BufferedWriter)+"\t");
         }
         System.out.println("Test BufferedWriter end.\n");

         System.out.println("Test NewOutputStream begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testNewOutputStream(lineNum,contentBytes);
             end = System.currentTimeMillis();
             long timeElapse_NewOutputStream = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_NewOutputStream)+"\t");
         }
         System.out.println("Test NewOutputStream end.\n");

         System.out.println("Test NewBufferedWriter begin.");
         bufferedWriter.write("\n");
         for (int lineNum = startlineNum; lineNum < maxlineNum; lineNum *= Multiplying) {
             begin = System.currentTimeMillis();
             testNewBufferedWriter(lineNum,contentChars);
             end = System.currentTimeMillis();
             long timeElapse_NewBufferedWriter = end - begin;
             bufferedWriter.write(String.valueOf(timeElapse_NewBufferedWriter)+"\t");
         }
         System.out.println("Test NewOutputStream end.\n");

         bufferedWriter.close();
     }

     /************************** I/O *****************************/
     //面向字节
     public static void testFileOutputStream (int lineNum, byte[] content) throws IOException {
         FileOutputStream fileOutputStream = new FileOutputStream(new File("./testFileOutputStream.txt"));
         while (--lineNum > 0) {
             fileOutputStream.write(content);
         }
         fileOutputStream.close();
     }

     public static void testBufferedOutputStream (int lineNum, byte[] content) throws IOException {
         FileOutputStream fileOutputStream = new FileOutputStream(new File("./testBufferedOutputStream.txt"));
         BufferedOutputStream bufferedOutputStream = new BufferedOutputStream(fileOutputStream);
         while (--lineNum > 0) {
             bufferedOutputStream.write(content);
         }
         bufferedOutputStream.close();
     }

     //面向字符
     public static void testFileWriter (int lineNum, String content) throws IOException {
         FileWriter fileWriter = new FileWriter("./testFileWriter.txt");
         while (--lineNum > 0) {
             fileWriter.write(content);
         }
         fileWriter.close();
     }

     public static void testBufferedWriter (int lineNum, String content) throws IOException {
         FileWriter fileWriter = new FileWriter("./testBufferedWriter.txt");
         BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);
         while (--lineNum > 0) {
             bufferedWriter.write(content);
         }
         bufferedWriter.close();
     }

     /************************** NIO ****************************/
     public static void testNewOutputStream (int lineNum, byte[] content) throws IOException {
         OutputStream outputStream = Files.newOutputStream(Paths.get("./testNewOutputStream.txt"));
         while (--lineNum > 0) {
             outputStream.write(content);
         }
         outputStream.close();
     }

     public static void testNewBufferedWriter (int lineNum,String content) throws IOException {
         BufferedWriter newBufferedReader = Files.newBufferedWriter(Paths.get("./testNewBufferedWriter.txt"));
         while (--lineNum > 0) {
             newBufferedReader.write(content);
         }
         newBufferedReader.close();
     }

     public static void main (String[] args) throws IOException {
         //多次测试时可清空result.txt文件
         FileWriter fileWriter = new FileWriter("./Result.txt");
         testDriver();
     }
 }

这次数据量就很大了，结果也就变了：

所以，数据量很小的情况下，字符到字节的编码操作带来的性能降低几乎忽略不计；而数据量很大的时候，编码耗费的时间就很可观了。至于为什么在小数据量的情况下FileWriter快很多，目前我认为是一次操作两个字节所致（有了缓存之后就差不多了）。

参考

Java IO流学习总结