求编写新Lucene Directory类型的简单示例代码

要实现自定义Lucene Directory，核心是继承org.apache.lucene.store.Directory抽象类，并实现其所有抽象方法。以下是一个基于内存的极简实现示例，可作为自定义Directory的模板：

核心实现代码 #

import org.apache.lucene.store.*;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class SimpleMemoryDirectory extends Directory {
    // 存储文件的映射表，键为文件名，值为内存输出流实例
    private final Map<String, SimpleMemoryIndexOutput> files = new HashMap<>();
    private long generation;

    @Override
    public String[] listAll() throws IOException {
        ensureOpen();
        synchronized (files) {
            return files.keySet().toArray(new String[0]);
        }
    }

    @Override
    public void deleteFile(String name) throws IOException {
        ensureOpen();
        synchronized (files) {
            files.remove(name);
        }
        generation++;
    }

    @Override
    public long fileLength(String name) throws IOException {
        ensureOpen();
        synchronized (files) {
            SimpleMemoryIndexOutput file = files.get(name);
            if (file == null) {
                throw new IOException("File not found: " + name);
            }
            return file.length();
        }
    }

    @Override
    public IndexOutput createOutput(String name, IOContext context) throws IOException {
        ensureOpen();
        synchronized (files) {
            files.remove(name);
            SimpleMemoryIndexOutput output = new SimpleMemoryIndexOutput(name);
            files.put(name, output);
            generation++;
            return output;
        }
    }

    @Override
    public IndexOutput createTempOutput(String prefix, String suffix, IOContext context) throws IOException {
        String tempName = prefix + "-" + generation + suffix;
        return createOutput(tempName, context);
    }

    @Override
    public void sync(Set<String> names) throws IOException {
        // 内存存储无需磁盘同步，空实现即可
    }

    @Override
    public void syncMetaData() throws IOException {
        // 元数据同步逻辑，此处空实现
    }

    @Override
    public IndexInput openInput(String name, IOContext context) throws IOException {
        ensureOpen();
        synchronized (files) {
            SimpleMemoryIndexOutput file = files.get(name);
            if (file == null) {
                throw new IOException("File not found: " + name);
            }
            return new SimpleMemoryIndexInput(file.getBytes());
        }
    }

    @Override
    protected void doClose() throws IOException {
        synchronized (files) {
            files.clear();
        }
    }

    // 自定义内存IndexOutput，基于Lucene ByteArrayDataOutput实现
    private static class SimpleMemoryIndexOutput extends IndexOutput {
        private final String name;
        private final ByteArrayDataOutput output = new ByteArrayDataOutput();

        public SimpleMemoryIndexOutput(String name) {
            super("SimpleMemoryIndexOutput(" + name + ")", "SIMPLE_MEMORY");
            this.name = name;
        }

        @Override
        public void writeByte(byte b) throws IOException {
            output.writeByte(b);
        }

        @Override
        public void writeBytes(byte[] b, int offset, int length) throws IOException {
            output.writeBytes(b, offset, length);
        }

        @Override
        public void flush() throws IOException {}

        @Override
        public void close() throws IOException {}

        @Override
        public long getFilePointer() {
            return output.getPosition();
        }

        @Override
        public long getChecksum() throws IOException {
            return output.getPosition(); // 简易校验和实现，实际需替换为逻辑校验
        }

        public byte[] getBytes() {
            return output.getBytes();
        }

        public long length() {
            return output.getPosition();
        }
    }

    // 自定义内存IndexInput，基于Lucene ByteArrayDataInput实现
    private static class SimpleMemoryIndexInput extends IndexInput {
        private final ByteArrayDataInput input;

        public SimpleMemoryIndexInput(byte[] bytes) {
            super("SimpleMemoryIndexInput", "SIMPLE_MEMORY");
            input = new ByteArrayDataInput(bytes);
        }

        @Override
        public byte readByte() throws IOException {
            return input.readByte();
        }

        @Override
        public void readBytes(byte[] b, int offset, int length) throws IOException {
            input.readBytes(b, offset, length);
        }

        @Override
        public long getFilePointer() {
            return input.getPosition();
        }

        @Override
        public void seek(long pos) throws IOException {
            input.setPosition((int) pos);
        }

        @Override
        public long length() {
            return input.length();
        }

        @Override
        public IndexInput slice(String sliceDescription, long offset, long length) throws IOException {
            byte[] sliceBytes = new byte[(int) length];
            int originalPos = input.getPosition();
            input.setPosition((int) offset);
            input.readBytes(sliceBytes, 0, (int) length);
            input.setPosition(originalPos);
            return new SimpleMemoryIndexInput(sliceBytes);
        }

        @Override
        public void close() throws IOException {}
    }
}

使用示例 #

import org.apache.lucene.store.IOContext;
import org.apache.lucene.store.IndexInput;
import org.apache.lucene.store.IndexOutput;
import java.io.IOException;

public class DirectoryUsageExample {
    public static void main(String[] args) throws IOException {
        // 自动关闭Directory资源
        try (Directory dir = new SimpleMemoryDirectory()) {
            // 写入文件
            try (IndexOutput output = dir.createOutput("sample.txt", IOContext.DEFAULT)) {
                output.writeString("Lucene自定义Directory示例");
            }

            // 读取文件内容
            try (IndexInput input = dir.openInput("sample.txt", IOContext.DEFAULT)) {
                String content = input.readString();
                System.out.println("读取内容：" + content);
            }

            // 列出目录下所有文件
            String[] allFiles = dir.listAll();
            System.out.println("目录文件列表：");
            for (String file : allFiles) {
                System.out.println("- " + file);
            }
        }
    }
}

关键说明 #

• 必须实现Directory的所有抽象方法，其中createOutput、openInput、listAll是核心操作
• 线程安全：用synchronized块保护文件映射表的读写操作
• 基于Lucene提供的ByteArrayDataOutput/ByteArrayDataInput简化内存IO实现，无需手动处理字节数组扩容
• 若要实现磁盘或其他存储介质的Directory，只需替换SimpleMemoryIndexOutput和SimpleMemoryIndexInput的底层存储逻辑即可

内容的提问来源于stack exchange，提问作者Vaibhav Khanduja