PrintWriter vs PrintStream vs OutputStreamWriter timecosts

Question

如您所知，我們在java中有幾個用於將數據寫入流的工具。
在這個示例代碼中，我通過運行時比較了它們。
有人可以解釋它嗎？謝謝。
下面是代碼：

import java.io.FileOutputStream; 
import java.io.OutputStreamWriter; 
import java.io.PrintStream; 
import java.io.PrintWriter; 

public class IOtests 
{ 

public static void main(String[] args) throws Exception 
{ 
    char[] chars = new char[100]; 
    byte[] bytes = new byte[100]; 
    for (int i = 0; i < 100; i++) 
    { 
     chars[i] = (char) i; 
     bytes[i] = (byte) i; 
    } 
    OutputStreamWriter out = new OutputStreamWriter(new FileOutputStream(
      "output.txt")); 
    long a = System.currentTimeMillis(); 
    for (int i = 0; i < 100000; i++) 
     for (char j : chars) 
      out.write(j); 
    System.out.println("OutputStreamWriter writing characters: " 
      + (System.currentTimeMillis() - a)); 
    out = new OutputStreamWriter(new FileOutputStream("output.txt")); 
    a = System.currentTimeMillis(); 
    for (int i = 0; i < 100000; i++) 
     for (byte j : bytes) 
      out.write(j); 
    System.out.println("OutputStreamWriter writing bytes: " 
      + (System.currentTimeMillis() - a)); 
    PrintStream out1 = new PrintStream("output.txt"); 
    a = System.currentTimeMillis(); 
    for (int i = 0; i < 100000; i++) 
     for (char j : chars) 
      out1.write(j); 
    System.out.println("PrintStream writing characters: " 
      + (System.currentTimeMillis() - a)); 
    out1 = new PrintStream("output.txt"); 
    a = System.currentTimeMillis(); 
    for (int i = 0; i < 100000; i++) 
     for (byte j : bytes) 
      out1.write(j); 
    System.out.println("PrintStream writing bytes: " 
      + (System.currentTimeMillis() - a)); 
    PrintWriter out2 = new PrintWriter("output.txt"); 
    a = System.currentTimeMillis(); 
    for (int i = 0; i < 100000; i++) 
     for (char j : chars) 
      out2.write(j); 
    System.out.println("PrintWriter writing characters: " 
      + (System.currentTimeMillis() - a)); 
    out1 = new PrintStream("output.txt"); 
    a = System.currentTimeMillis(); 
    for (int i = 0; i < 100000; i++) 
     for (byte j : bytes) 
      out2.write(j); 
    System.out.println("PrintWriter writing bytes: " 
      + (System.currentTimeMillis() - a)); 
} 

} 

結果：

OutputStreamWriter書寫字符：4141個
OutputStreamWriter寫入字節：3546個
的PrintStream書寫字符：86516個
的PrintStream寫入字節：70484
PrintWriter寫人物：938
PrintWriter寫作字節：2484

請注意，所有時間都是以毫秒爲單位。

Answer 1

我已經減少了你的問題的本質：

public class Test { 
    static byte[] bytes = new byte[10_000_000]; 
    static { 
    for (int i = 0; i < bytes.length; i++) bytes[i] = (byte) (i%100+32); 
    } 
    public static void main(String[] args) throws Exception { 
    writer(true); 
    writer(false); 
    stream(true); 
    stream(false); 
    } 

    static void writer(boolean flush) throws IOException { 
    Writer out = new FileWriter("output.txt"); 
    long a = System.currentTimeMillis(); 
    for (byte j : bytes) { 
     out.write(j); 
     if (flush) out.flush(); 
    } 
    out.close(); 
    System.out.println("FileWriter with" + (flush? "":"out") + " flushing: " + 
     (System.currentTimeMillis() - a)); 
    } 
    static void stream(boolean flush) throws IOException { 
    OutputStream out = new FileOutputStream("output.txt"); 
    long a = System.currentTimeMillis(); 
    for (byte j : bytes) { 
     out.write(j); 
     if (flush) out.flush(); 
    } 
    out.close(); 
    System.out.println("FileOutputStream with" + (flush? "":"out") + " flushing: " + 
     (System.currentTimeMillis() - a)); 
    } 
} 

注：

• 完成時正確關閉的資源;
• 雙循環被單循環取代，而是一個更大的數組;
• 避免編寫控制字符來逃避自動刷新行爲;
• 只使用字節數組，因爲您在所有情況下只測試一種方法：write(int)。因此，使用字節還是字符無關緊要;
• 刪除了除FileWriter和FileOutputStream之外的所有內容，因爲所有其他情況歸結爲這兩個;
• 以兩種模式測試寫入器和輸出流：在每次寫入之後刷新，直到關閉時才刷新。

現在，當你運行它，你會得到類似以下的輸出：

FileWriter with flushing: 28235 
FileWriter without flushing: 828 
FileOutputStream with flushing: 23984 
FileOutputStream without flushing: 23641 

那麼，有什麼教訓？

• 所有的寫入程序都被緩衝，因爲它們在內部被委託給本身被緩衝的StreamEncoder;
• FileOutputStream未被緩衝;
• 非緩衝寫入字節一個字節是非常慢的。

良好做法的需求，你總是做寫入緩存：要麼使用緩衝片，或維持在您身邊一個明確的緩衝。