如果该类具有每个字段的默认构造函数和设置器,OpenCSV 会愉快地将记录转换为对象。但是,我希望为其生成对象的类是使用最终字段、私有构造函数和生成器定义的。例如,如果我想创建一个类型为
X
的对象,其中 X
是由 定义的
public class X {
private final String val;
private X(final String val) { this.val = val; }
public Builder builder() { return new Builder(); }
public static class Builder {
private String val;
public Builder withVal(final String val) { this.val = val; return this; }
public X build() { return new X(val); }
}
}
我已经看到了
com.opencsv.bean.MappingStrategy
界面,想知道这是否可以以某种方式应用,但我还没有找到解决方案。有什么想法吗?
com.opencsv.bean.MappingStrategy
看起来是处理这个问题的好方法,但是由于您有最终属性和私有构造函数,因此您需要从 java 反射系统获得一些帮助。
OpenCSV 的内置映射策略继承自
AbstractMappingStrategy
类,使用无参数默认构造函数构造 bean,然后查找 setter 方法来填充值。
可以准备一个
MappingStrategy
实现,它会找到一个合适的构造函数,其参数与 CSV 文件中映射列的顺序和类型相匹配,并使用它来构造 bean。即使构造函数是私有的或受保护的,也可以通过反射系统访问它。
例如以下 CSV:
number;string
1;abcde
2;ghijk
可以映射到以下类:
public class Something {
@CsvBindByName
private final int number;
@CsvBindByName
private final String string;
public Something(int number, String string) {
this.number = number;
this.string = string;
}
// ... getters, equals, toString etc. omitted
}
使用以下 CvsToBean 实例:
CsvToBean<Something> beanLoader = new CsvToBeanBuilder<Something>(reader)
.withType(Something.class)
.withSeparator(';')
.withMappingStrategy(new ImmutableClassMappingStrategy<>(Something.class))
.build();
List<Something> result = beanLoader.parse();
ImmutableClassMappingStrategy
的完整代码:
import com.opencsv.bean.AbstractBeanField;
import com.opencsv.bean.BeanField;
import com.opencsv.bean.HeaderColumnNameMappingStrategy;
import com.opencsv.exceptions.CsvConstraintViolationException;
import com.opencsv.exceptions.CsvDataTypeMismatchException;
import com.opencsv.exceptions.CsvRequiredFieldEmptyException;
import java.beans.IntrospectionException;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
/**
* A {@link com.opencsv.bean.MappingStrategy} implementation which allows to construct immutable beans containing
* final fields.
*
* It tries to find a constructor with order and types of arguments same as the CSV lines and construct the bean using
* this constructor. If not found it tries to use the default constructor.
*
* @param <T> Type of the bean to be returned
*/
public class ImmutableClassMappingStrategy<T> extends HeaderColumnNameMappingStrategy<T> {
/**
* Constructor
*
* @param type Type of the bean which will be returned
*/
public ColumnMappingStrategy(Class<T> type) {
setType(type);
}
@Override
public T populateNewBean(String[] line) throws InstantiationException, IllegalAccessException, IntrospectionException, InvocationTargetException, CsvRequiredFieldEmptyException, CsvDataTypeMismatchException, CsvConstraintViolationException {
verifyLineLength(line.length);
try {
// try constructing the bean using explicit constructor
return constructUsingConstructorWithArguments(line);
} catch (NoSuchMethodException e) {
// fallback to default constructor
return super.populateNewBean(line);
}
}
/**
* Tries constructing the bean using a constructor with parameters for all matching CSV columns
*
* @param line A line of input
*
* @return
* @throws NoSuchMethodException in case no suitable constructor is found
*/
private T constructUsingConstructorWithArguments(String[] line) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
Constructor<? extends T> constructor = findSuitableConstructor(line.length);
// in case of private or protected constructor, try to set it to be accessible
if (!constructor.canAccess(null)) {
constructor.setAccessible(true);
}
Object[] arguments = prepareArguments(line);
return constructor.newInstance(arguments);
}
/**
* Tries to find a suitable constructor with exact number and types of parameters in order defined in the CSV file
*
* @param columns Number of columns in the CSV file
* @return Constructor reflection
* @throws NoSuchMethodException in case no such constructor exists
*/
private Constructor<? extends T> findSuitableConstructor(int columns) throws NoSuchMethodException {
Class<?>[] types = new Class<?>[columns];
for (int col = 0; col < columns; col++) {
BeanField<T> field = findField(col);
Class<?> type = field.getField().getType();
types[col] = type;
}
return type.getDeclaredConstructor(types);
}
/**
* Prepare arguments with correct types to be used in the constructor
*
* @param line A line of input
* @return Array of correctly typed argument values
*/
private Object[] prepareArguments(String[] line) {
Object[] arguments = new Object[line.length];
for (int col = 0; col < line.length; col++) {
arguments[col] = prepareArgument(col, line[col], findField(col));
}
return arguments;
}
/**
* Prepare a single argument with correct type
*
* @param col Column index
* @param value Column value
* @param beanField Field with
* @return
*/
private Object prepareArgument(int col, String value, BeanField<T> beanField) {
Field field = beanField.getField();
// empty value for primitive type would be converted to null which would throw an NPE
if ("".equals(value) && field.getType().isPrimitive()) {
throw new IllegalArgumentException(String.format("Null value for primitive field '%s'", headerIndex.getByPosition(col)));
}
try {
// reflectively access the convert method, as it's protected in AbstractBeanField class
Method convert = AbstractBeanField.class.getDeclaredMethod("convert", String.class);
convert.setAccessible(true);
return convert.invoke(beanField, value);
} catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
throw new IllegalStateException(String.format("Unable to convert bean field '%s'", headerIndex.getByPosition(col)), e);
}
}
}
另一种方法可能是将 CSV 列映射到构建器本身,然后构建不可变类。
oujesky 的响应是一个很好的方法,但需要一个构造函数,其中参数的顺序与 CSV 文件中的顺序完全匹配,但可用于其他注入任务。
但是,要使类不可变,您可以将 setter 方法的访问级别设置为私有。下面的示例使用 opencsv 中的 CsvToBean 函数。
@Getter
public class X {
private final int a;
private X(int a) {
this.a = a;
}
@Setter(AccessLevel.PRIVATE)
public static class XBuilder {
@CsvBindByName
private int a;
public X build() {
return new X(a);
}
}
}
@Test
void simpleTest() {
String testData = """
a
1337
""";
List<X> xs = new CsvToBeanBuilder<X.XBuilder>(new StringReader(testData))
.withType(X.XBuilder.class)
.build()
.stream()
.map(X.XBuilder::build)
.collect(Collectors.toList());
Assertions.assertEquals(1, xs.size());
Assertions.assertEquals(1337, xs.get(0).getA());
}