如何通过其注释获取类的字段

如果需要进行多次访问，则可以预处理注释。

public class ColumnExtracter<T> {    private final Map<String, Field> fieldsByColumn;    public ColumnExtracter(Class<T> clazz) {        this.fieldsByColumn = Stream.of(clazz.getDeclaredFields())   .filter(field -> field.isAnnotationPresent(Column.class))   .collect(Collectors.toMap(field -> field.getAnnotation(Column.class).name(), Function.identity()));    }    public Field getColumnField(String columnName) {        return fieldsByColumn.get(columnName);    }    public <R> R extract(String columnName, T t, Class<R> clazz) throws IllegalAccessException {        return clazz.cast(extract(columnName, t));    }    public Object extract(String columnName, T t) throws IllegalAccessException {        return getColumnField(columnName).get(t);    }}