第一：一个罕见的需求，通常是相反的，或者有一个filechannel，所以可以使用bytebuffer。
pipedinputstream是可能的，在第二个线程中启动pipedoutputstream。然而，这是不必要的。
读书人给人以魅力。unicode代码点是从一个或两个字符（后者是代理项对）派生的。

/**
 * Reader for an InputSteam of UTF-8 text bytes.
 */
public class ReaderInputStream extends InputStream {

    private final Reader reader;
    private boolean eof;
    private int byteCount;
    private byte[] bytes = new byte[6];

    public ReaderInputStream(Reader reader) {
        this.reader = reader;
    }

    @Override
    public int read() throws IOException {
        if (byteCount > 0) {
            int c = bytes[0];
            --byteCount;
            for (int i = 0; i < byteCount; ++i) {
                bytes[i] = bytes[i + 1];
            }
            return c;
        }
        if (eof) {
            return -1;
        }

        int c = reader.read();
        if (c == -1) {
            eof = true;
            return -1;
        }
        char ch = (char) c;
        String s;
        if (Character.isHighSurrogate(ch)) {
            c = reader.read();
            if (c == -1) {
                // Error, low surrogate expected.
                eof = true;
                //return -1;
                throw new IOException("Expected a low surrogate char i.o. EOF");
            }
            char ch2 = (char) c;
            if (!Character.isLowSurrogate(ch2)) {
                throw new IOException("Expected a low surrogate char");
            }
            s = new String(new char [] {ch, ch2});
        } else {
            s = Character.toString(ch);
        }
        byte[] bs = s.getBytes(StandardCharsets.UTF_8);
        byteCount = bs.length;
        System.arraycopy(bs, 0, bytes, 0, byteCount);
        return read();
    }
}

        Path source = Paths.get("...");
        Path target = Paths.get("...");
        try (Reader reader = Files.newBufferedReader(source, StandardCharsets.UTF_8);
                InputStream in = new ReaderInputStream(reader)) {
            Files.copy(in, target);
        }

使用apache commons io库，您可以在一行中完成此转换：

//import org.apache.commons.io.input.ReaderInputStream;

    InputStream inputStream = new ReaderInputStream(reader, StandardCharsets.UTF_8);

你可以在网上阅读这节课的文件https://commons.apache.org/proper/commons-io/apidocs/org/apache/commons/io/input/readerinputstream.html
也许值得一试，看看它是否也解决了内存问题。