java - 处理来自 Java 输入流的下一个字符(完整的 unicode 代码点)

Question

我需要逐个字符地解析 UTF-8 输入流(UTF-8 代码点，而不是 Java 的字符)。什么是最好的方法？

更新问题以使其更清楚(感谢@skomisa):所以下面的非流:

private static String replaceNonBPMWithUnknownCharacter(final String input) {
    StringBuilder result = new StringBuilder(input.length());
    input.codePoints().forEach((codePoint) -> {
        if (isBmpCodePoint(codePoint)) {
            result.append('\ufffd');
        } else {
            result.append(isBmpCodePoint(codePoint) ? toChars(codePoint) : REPLACEMENT_CHAR);
        }
    });
    return result.toString();
}


String result = replaceNonBPMWithUnknownCharacter("\uD83D\uDE0E? X")

我想要一个流媒体版本，例如:

InputStream stream = replaceNonBPMWithUnknownCharacter(new ByteArrayInputStream("\uD83D\uDE0E? Y".getBytes(UTF_8)))

使用尽可能少的 CPU 和内存。以下问题是类似的，但不是流式的:Read next character (full unicode code point) from Java input stream .

最重要的是:如何从流中读取代码点？ (那么如何将我知道它们是 UTF-8 编码的字节流转换为代码点流)。

Answer 1

首先要注意的是:

• UTF-8 字符可以由 1、2、3 或 4 字节序列组成。
• 字符中的字节数由第一个(或唯一)字节中的某些位设置决定。请参阅 Unicode Specification 中的表 3.6 UTF-8 位分布了解详情。

所以整体的做法是:

• 读取第一个字节并检查其位模式以确定第一个字符中有多少字节。
• 读取第一个字符所需的任何后续字节。
• 创建 String基于组成该字符的字节，然后调用 String.codePointAt()获取其代码点。
• 将该代码点添加到 List<Integer> .
• 对后续字节重复前面的所有步骤，直到 EOF。
• 返回List<Integer>作为代码点流。

这是代码，使用一些可变字节长度的随机 Unicode 字符作为数据:

import java.io.ByteArrayInputStream;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Stream;
import static java.nio.charset.StandardCharsets.UTF_8;

public class Main {

    public static void main(String[] args) {
        String text = "¢\uD841\uDF31\u30918औWあش";
        Stream<Integer> codePoints = Main.processByteStream(new ByteArrayInputStream(text.getBytes(UTF_8)));
        codePoints.forEach(System.out::println);
    }

    /**
     * Processes a stream of bytes, and returns a Stream of Unicode codepoints
     * associated with the characters derived from that byte stream.
     *
     * @param bais ByteArrayInputStream to be processed.
     * @return A stream of Unicode codepoints derived from UTF-8 characters in the supplied stream.
     */
    private static Stream<Integer> processByteStream(ByteArrayInputStream bais) {

        int nextByte = 0;
        byte b = 0;
        byte[] utf8Bytes = null;
        int byteCount = 0;
        List<Integer> codePoints = new ArrayList<>();

        while ((nextByte = bais.read()) != -1) {
            b = (byte) nextByte;
            byteCount = Main.getByteCount(b);
            utf8Bytes = new byte[byteCount];
            utf8Bytes[0] = (byte) nextByte;
            for (int i = 1; i < byteCount; i++) { // Get any subsequent bytes for this UTF-8 character.
                nextByte = bais.read();
                utf8Bytes[i] = (byte) nextByte;
            }
            int codePoint = new String(utf8Bytes, StandardCharsets.UTF_8).codePointAt(0);
            codePoints.add(codePoint);
        }
        return codePoints.stream();
    }

    /**
     * Returns the number of bytes in a UTF-8 character based on the bit pattern
     * of the supplied byte. The only valid values are 1, 2 3 or 4. If the
     * byte has an invalid bit pattern an IllegalArgumentException is thrown.
     *
     * @param b The first byte of a UTF-8 character.
     * @return The number of bytes for this UTF-* character.
     * @throws IllegalArgumentException if the bit pattern is invalid.
     */
    private static int getByteCount(byte b) throws IllegalArgumentException {
        if ((b >= 0)) return 1;                                             // Pattern is 0xxxxxxx.
        if ((b >= (byte) 0b11000000) && (b <= (byte) 0b11011111)) return 2; // Pattern is 110xxxxx.
        if ((b >= (byte) 0b11100000) && (b <= (byte) 0b11101111)) return 3; // Pattern is 1110xxxx.
        if ((b >= (byte) 0b11110000) && (b <= (byte) 0b11110111)) return 4; // Pattern is 11110xxx.
        throw new IllegalArgumentException(); // Invalid first byte for UTF-8 character.
    }
}

这是运行它的输出。它仅列出返回的代码点 Stream<Integer> :

C:\Java\openJDK\jdk-11.0.2\bin\java.exe -javaagent:C:\Users\johndoe\AppData\Local\JetBrains\Toolbox\apps\IDEA-U\ch-0\191.4738.6\lib\idea_rt.jar=60544:C:\Users\johndoe\AppData\Local\JetBrains\Toolbox\apps\IDEA-U\ch-0\191.4738.6\bin -Dfile.encoding=UTF-8 -classpath C:\Users\johndoe\IdeaProjects\Codepoint\out\production\Codepoint Main
162
132913
12433
56
2324
87
12354
1588

Process finished with exit code 0

注意事项:

• 可以直接从字符的字节派生代码点，但是it is a convoluted process .打电话String.codePointAt()是一种效率较低但更清洁的替代方法。
• 我无法生成无效数据。似乎任何无效字节都以某种方式转换为 U+FFFD(十进制 65533)，the replacement character , 所以抛出 IllegalArgumentException可能没有必要。