java里面缓冲流里面缓冲区满了不会自动flush写入文件

Question

问下为啥我这char数组大小都超过缓冲区2个字节的大小了，我看文件还没有自动写入，这是为啥？缓冲区不是满了就会自动flush吗，就会写入到文件嘛？

Answer 1

你设置缓存大小为2，只是表示你的显示缓存BufferedWriter失效应该刷新，在最底层有个默认的8192缓存，你的数据不够所以没有刷出.如果你一定要看下效果的话可以运行看下效果，就是你要的预期了

import lombok.SneakyThrows;
import sun.nio.cs.StreamEncoder;

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.OutputStreamWriter;
import java.lang.reflect.Field;
import java.nio.ByteBuffer;

public class Test03 {
    @SneakyThrows
    public static void main(String[] args) {
        final FileWriter fileWriter = new FileWriter("test.txt");
        changeBufferTo2(fileWriter);
        final BufferedWriter bufferedWriter = new BufferedWriter(fileWriter, 2);
        final char[] chars = {'8', '2', '3', '4', '5'};
        bufferedWriter.write(chars);
    }

    private static void changeBufferTo2(FileWriter fileWriter) throws NoSuchFieldException, IllegalAccessException {
        final Field se = OutputStreamWriter.class.getDeclaredField("se");
        se.setAccessible(true);
        final StreamEncoder o = (StreamEncoder) se.get(fileWriter);
        final Field bb = o.getClass().getDeclaredField("bb");
        bb.setAccessible(true);
        bb.set(o, ByteBuffer.allocate(2));
    }
}