衆所周知，Java中有多種針對檔案的操作類，以面向位元組流和字元流可分為兩大類，這裡以寫入為例：
面向位元組流的：FileOutputStream和BufferedOutputStream
面向字元流的：FileWriter和BufferedWriter
近年來發展出New I/O ，也叫NIO，裡面又包裝了兩個類：NewOutputStream 和 NewBufferedWriter。現在，我們建立測試程式，比較這些類寫入檔案的性能。
機器配置
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
測試程式
縱向比較：幾種檔案操作類向檔案中寫入相同行數的内容（每行内容均為“寫入檔案Data\n”），比較其耗費時間。
橫向比較：對于同一個檔案操作類，比較寫入不同行數内容情況下所耗費時間；本文以2的次方指數級增長行數。
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;//寫入檔案的最大行數
int startlineNum = 1;//寫入檔案的行數
int Multiplying = 2;//行數增長倍率
long begin = 0L;
long end = 0L;
//将時間統計寫入檔案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 *****************************/
//面向位元組
public static void testFileOutputStream (int lineNum) throws IOException {
FileOutputStream fileOutputStream = new FileOutputStream(new File("./testFileOutputStream.txt"));
while (--lineNum > 0) {
fileOutputStream.write("寫入檔案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("寫入檔案Data\n".getBytes());
}
bufferedOutputStream.close();
}
//面向字元
public static void testFileWriter (int lineNum) throws IOException {
FileWriter fileWriter = new FileWriter("./testFileWriter.txt");
while (--lineNum > 0) {
fileWriter.write("寫入檔案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("寫入檔案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("寫入檔案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("寫入檔案Data\n");
}
newBufferedReader.close();
}
public static void main (String[] args) throws IOException {
//多次測試時可清空result.txt檔案
FileWriter fileWriter = new FileWriter("./Result.txt");
testDriver();
}
}
測試結果，如圖所示：
從上圖可以看出，寫入行數超過20W以上時，FileOutputStream和NewOutputStream耗費時間遠遠超出其他4個類。為了清晰，讓我們放大其他4個類的圖：
可以看出，這4個類中，BufferWriter和NewBufferedWriter所耗費時間更少，但總體差别不是很大。
讓我們再來看看，寫入26W行資料以下時的情況：
可以看出，在資料量較小的情況下，這4個類所耗費時間的差異并不是很大，在更小的資料量下，它們的效率幾乎沒有差别。
後記
從以上分析可知（注意橫坐标寫入行數是指數級增加的），各個類的時間複雜度大緻為O(k)，其中不同的類的k不同，導緻了最終巨大的差異。
這裡隻給出了測試結果，并未分析其底層實作原理，歡迎評論區留言。
另外，我沒有在其他機器測試，有興趣的小夥伴可以将自己的測試結果發出來，共同進步^_^。
附件
本次測試資料結果(若看不清，可以将浏覽器字型放大，或下載下傳到本地看)
~~~~~~~~~~分割線~~~~~~~~~
評論區小夥伴“ andorxor”提出：
XXXOutputStream是用來寫二進制的，你把字元串轉換成位元組數組再寫自然就慢了，主要慢在轉換的過程。
是以，将程式修改，提前把字元和位元組内容都準備好，再次驗證。新程式如下：
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;//寫入檔案的最大行數
int startlineNum = 1;//寫入檔案的行數
int Multiplying = 2;//行數增長倍率
String contentChars = "寫入檔案Data\n";//每行的内容（字元流）
byte[] contentBytes = "寫入檔案Data\n".getBytes();//每行的内容（位元組流）
long begin = 0L;
long end = 0L;
//将時間統計寫入檔案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 *****************************/
//面向位元組
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();
}
//面向字元
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 {
//多次測試時可清空result.txt檔案
FileWriter fileWriter = new FileWriter("./Result.txt");
testDriver();
}
}
結果如圖所示：
可以看出和前面的案例幾乎沒有差異（圖就不畫了）。
是以XXXOutputStream效率低的原因并不是字元串轉換成位元組數組，而是其本身的實作方式所緻。
作者：xiaoxi666