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这个示例代码中，我通过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写字符：4141
  OutputStreamWriter写入字节：3546
  PrintStream写字符：86516
  PrintStream写入字节：70484
  PrintWriter写字：938
  PrintWriter写入字节：2484

请注意，所有时间都以毫秒为单位。

Answer 1

我已将你的问题简化为其本质：

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));
  }
}

注意：

完成后正确关闭资源;
双循环替换为单循环，但是更大的数组;
避免编写控制字符以逃避autoflush行为;
仅使用字节数组，因为在所有情况下只测试一种方法：write(int)。因此，无论使用字节还是字符，都没有区别;
删除了FileWriter和FileOutputStream以外的所有内容，因为所有其他情况归结为这两个;
以两种模式测试编写器和输出流：每次写入后刷新，并且在关闭之前不要冲洗。

现在，当你运行它时，你会得到如下输出：

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

那么，教训是什么？

所有编写器都是缓冲的，因为它们在内部委托给StreamEncoder，它本身是缓冲的;
FileOutputStream未缓冲;
逐字节非缓冲写入非常慢。

良好实践要求您始终执行缓冲写入：使用缓冲接收器，或者在您身边维护显式缓冲区。

PrintWriter vs PrintStream vs OutputStreamWriter timecosts

1 个答案: