如何升级使用Apache Commons FileUpload DiskFileItem适配Jakarta命名空间？

由于FileUpload v2将DiskFileItem设为final类，无法通过继承扩展功能，你可以用装饰器模式替代继承，实现FileItem接口并内部持有DiskFileItem实例，既保留原有监听逻辑，又不需要修改框架源码。

修改后的代码如下：

import org.apache.commons.fileupload2.core.DiskFileItem;
import org.apache.commons.fileupload2.core.FileItem;
import java.io.IOException;
import java.io.OutputStream;
import java.io.File;
import java.io.InputStream;
import java.io.UnsupportedEncodingException;

public class MonitoredDiskFileItem implements FileItem {
    private final DiskFileItem delegate;
    private final OutputStreamListener listener;
    private MonitoredOutputStream mos = null;

    public MonitoredDiskFileItem(String fieldName, String contentType, boolean isFormField, String fileName, int sizeThreshold, File repository, OutputStreamListener listener) {
        this.delegate = new DiskFileItem(fieldName, contentType, isFormField, fileName, sizeThreshold, repository);
        this.listener = listener;
    }

    // 以下为FileItem接口方法的委托实现，直接调用delegate的对应方法
    @Override
    public String getFieldName() {
        return delegate.getFieldName();
    }

    @Override
    public void setFieldName(String name) {
        delegate.setFieldName(name);
    }

    @Override
    public boolean isFormField() {
        return delegate.isFormField();
    }

    @Override
    public void setFormField(boolean state) {
        delegate.setFormField(state);
    }

    @Override
    public String getContentType() {
        return delegate.getContentType();
    }

    @Override
    public String getName() {
        return delegate.getName();
    }

    @Override
    public InputStream getInputStream() throws IOException {
        return delegate.getInputStream();
    }

    @Override
    public long getSize() {
        return delegate.getSize();
    }

    @Override
    public byte[] get() {
        return delegate.get();
    }

    @Override
    public String getString(String encoding) throws UnsupportedEncodingException {
        return delegate.getString(encoding);
    }

    @Override
    public String getString() {
        return delegate.getString();
    }

    @Override
    public void write(File file) throws Exception {
        delegate.write(file);
    }

    @Override
    public void delete() {
        delegate.delete();
    }

    @Override
    public String getContentType(String defaultContentType) {
        return delegate.getContentType(defaultContentType);
    }

    @Override
    public File getStoreLocation() {
        return delegate.getStoreLocation();
    }

    @Override
    public boolean isInMemory() {
        return delegate.isInMemory();
    }

    // 仅重写需要扩展的getOutputStream方法，保留原有监听逻辑
    @Override
    public OutputStream getOutputStream() throws IOException {
        if (mos == null) {
            mos = new MonitoredOutputStream(delegate.getOutputStream(), listener);
        }
        return mos;
    }
}

方案说明 #

• 无需修改FileUpload源码，完全基于官方API实现，避免了自定义编译框架带来的兼容性风险
• 核心逻辑和原有代码一致：仅在获取输出流时包装为带监听的MonitoredOutputStream
• 所有未修改的功能直接委托给内部的DiskFileItem实例，保证原有行为不变
• 代码结构清晰，后续维护成本低，符合框架将DiskFileItem设为final的设计意图（限制继承，鼓励组合扩展）

内容的提问来源于stack exchange，提问作者Borea Deitz