java - PrintWriter vs PrintStream vs OutputStreamWriter 时间成本

Question

如您所知，我们在 java 中有多种工具可以将数据写入流。
在此示例代码中，我按运行时对它们进行了比较。
有人可以准确地解释一下吗？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 写入字符：4141
OutputStreamWriter 写入字节：3546
PrintStream 写入字符：86516
PrintStream 写入字节：70484
PrintWriter 写入字符：938
PrintWriter 写入字节：2484

请注意，所有时间都以毫秒为单位。

Answer 1

I've reduced your question to its essence:

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));
  }
}

Notes:

• properly closing the resources when done;
• double loop replaced by single loop, but a larger array;
• avoid writing control characters to evade autoflush behavior;
• only using byte array since you are testing only one method in all cases: write(int). Therefore it makes no difference whether you are using bytes or chars;
• removed everything except a FileWriter and a FileOutputStream because all other cases boil down to these two;
• testing both writer and output stream in two modes: flush after each write, and don't flush at all until close.

Now, when you run this, you'll get output like the following:

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

So, what's the lesson?

• all writers are buffered because internally they delegate to StreamEncoder which is itself buffered;
• FileOutputStream is not buffered;
• non-buffered writing byte-by-byte is very slow.

Good practices demand that you always do buffered writing: either using buffered sinks, or maintaining an explicit buffer on your side.