PrintWriter vs PrintStream vs OutputStreamWriter timecosts

Question

如您所知,我们在java中有几个用于将数据写入流的工具.
在此示例代码中,我按运行时对它们进行了比较.
有人可以解释一下吗？谢谢.
这是代码:

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));
}

}

结果:

OutputStreamWriter练字:4141个
OutputStreamWriter写入字节:3546
PrintStream的文字符号:86516
PrintStream的字节写入:70484
的PrintWriter练字:938个
的PrintWriter写入字节:2484

请注意,所有时间都以毫秒为单位.

Answer 1

我已将您的问题简化为本质：

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));
  }
}

笔记：

• 完成后正确关闭资源；
• 双循环替换为单循环，但数组更大；
• 避免编写控制字符来逃避自动刷新行为；
• 仅使用字节数组，因为您在所有情况下都只测试一种方法：write(int). 因此，无论您使用字节还是字符都没有区别；
• 删除除了 aFileWriter和 a之外的所有内容，FileOutputStream因为所有其他情况都归结为这两个；
• 以两种模式测试写入器和输出流：每次写入后刷新，并且在关闭之前完全不刷新。

现在，当您运行它时，您将获得如下输出：

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

那么，教训是什么？

• 所有写入器都被缓冲，因为它们在内部委托给StreamEncoder它本身被缓冲；
• FileOutputStream 没有缓冲；
• 非缓冲逐字节写入非常慢。

好的做法要求您始终进行缓冲写入：要么使用缓冲接收器，要么在您身边维护一个显式缓冲区。