PrintWriter与PrintStream与OutputStreamWriter的时间成本

Question

正如您所知，我们在java中有几个工具可以将数据写入流中
在这个示例代码中，我按运行时对它们进行了比较
有人能解释清楚吗？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写入字符：4141
OutputStreamWriter写入字节数：3546
PrintStream写入字符：86516
PrintStream写入字节数：70484
PrintWriter书写字符：938
PrintWriter写入字节数：2484

请注意，所有时间都以毫秒为单位。

Answer 1

我已经把你的问题归结为本质：

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));
  }
}

注：

• 完成后适当关闭资源
• 双循环被单循环取代，但数组更大
• 避免编写控制字符以避免自动刷新行为
• 只使用字节数组，因为在所有情况下只测试一种方法：write(int)。因此，无论您使用的是字节还是字符，都没有区别
• 删除了除FileWriter和FileOutputStream之外的所有情况，因为所有其他情况都归结为这两种情况
• 以两种模式测试写入程序和输出流：每次写入后刷新，直到关闭才刷新

现在，当你运行这个程序时，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，教训是什么呢？

• 所有写入器都被缓冲，因为它们在内部委托给本身被缓冲的CCD_ 4
• CCD_ 5不被缓冲
• 非缓冲的逐字节写入非常慢

良好的实践要求始终执行缓冲写入：要么使用缓冲接收器，要么在您一侧维护显式缓冲区。