ModelMapper：确保该方法的参数为零，并且不返回void
我为模型映射器配置了以下配置，以将User类的实例转换为ExtendedGetUserDto的实例。
public ExtendedGetUserDto convertToExtendedDto(User user) { PropertyMap userMap = new PropertyMap() { protected void configure() { map().setDescription(source.getDescription()); map().setId(source.getId()); // map().setReceivedExpenses( // source.getReceivedExpenses() // .stream() // .map(expense -> expenseDtoConverter.convertToDto(expense)) // .collect(Collectors.toSet()) // ); Set result = new HashSet(); for (Invitation inv: source.getReceivedInvitations()) { System.out.println("HELLO"); //result.add(null); } //map().setReceivedInvitations(result); } }; modelMapper.addMappings(userMap); return modelMapper.map(user, ExtendedGetUserDto.class); }
在注释掉setReceivedExpense之前我收到了这个错误：
org.modelmapper.ConfigurationException: ModelMapper configuration errors: 1) Invalid source method java.util.stream.Stream.map(). Ensure that method has zero parameters and does not return void. 2) Invalid source method java.util.stream.Stream.collect(). Ensure that method has zero parameters and does not return void. 2 errors
在花了一些时间而没有找到根本原因之后，我试图删除DTO中所有可疑的循环依赖（我在GetExpenseDto引用了GetUserDto ，使用了GetExpenseDto的返回结果）我仍然收到相同的错误，我注释掉了map().setReceivedExpenses （正如你在代码中看到的那样）并用简单的for循环替换它。
我收到以下错误：
1) Invalid source method java.io.PrintStream.println(). Ensure that method has zero parameters and does not return void.
为什么我会收到这些错误？
编辑1
User.java
@Entity @Table(name="User") public class User implements Serializable { private static final long serialVersionUID = 1L; @Id @GeneratedValue(strategy = GenerationType.IDENTITY) @Column(name="id") private long id; @Column(name = "name") private String name; @Size(min=15, max=15) @Column(name="image_id") private String imageId; @Size(max=100) @Column(name="description") private String description; @OneToMany(mappedBy="admin") private Set ownedGroups; @ManyToMany(mappedBy="members") private Set memberGroups; @OneToMany(cascade = CascadeType.ALL, fetch=FetchType.EAGER, mappedBy="owner") private Set ownedExpenses; @ManyToMany(cascade = CascadeType.REFRESH, fetch=FetchType.EAGER) private Set receivedExpenses; @OneToMany(cascade=CascadeType.ALL) private Set ownedInvitations; @OneToMany(cascade=CascadeType.ALL) private Set receivedInvitations; //setters and getters for attributes }
ExtendedGetUserDto.java
public class ExtendedGetUserDto extends GetUserDto { private static final long serialVersionUID = 1L; private Set receivedInvitations; private Set receivedExpenses; private Set ownedExpenses; private Set ownedGroups; private Set memberGroups; //setters and getters for attributes }
您收到这些错误是因为PropertyMap限制了您在configure()可以执行的操作 。
在Javadoc中 ：
PropertyMap使用嵌入式域特定语言（EDSL）来定义源和目标方法和值如何相互映射。 Mapping EDSL允许您使用引用要映射的源和目标属性的实际代码来定义映射。 EDSL的用法在以下实施例中说明。
从技术上讲，它涉及字节码分析，操作和代理，并且它需要适合此EDSL的Java方法调用。 这个聪明的技巧允许ModelMapper记录您的映射指令，并随意重放它们 。
要了解库源代码：您得到的错误是invalidSourceMethod ， 在ExplicitMappingVisitor中抛出此处，其中ObjectMapper访问并使用ASM库检测 configure方法的代码。
以下示例是一个独立的可运行示例，应该有助于澄清。 我邀请您在ModelMapperTest.java复制它并实际运行它，然后在configure()切换注释以重现错误：
import org.modelmapper.ModelMapper; import org.modelmapper.PropertyMap; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class ModelMapperTest { public static void main(String[] args) { PropertyMap propertyMap = new PropertyMap() { protected void configure() { /* This is executed exactly ONCE, to "record" the mapping instructions. * The bytecode of this configure() method is analyzed to produce new mapping code, * a new dynamically-generated class with a method that will basically contain the same instructions * that will be "replayed" each time you actually map an object later. * But this can only work if the instructions are simple enough (ie follow the DSL). * If you add non-compliant code here, it will break before "configure" is invoked. * Non-compliant code is supposedly anything that does not follow the DSL. * In practice, the framework only tracks what happens to "map()" and "source", so * as long as print instructions do not access the source or target data (like below), * the framework will ignore them, and they are safe to leave for debug. */ System.out.println("Entering configure()"); // This works List things = source.getThings(); map().setThingsCSVFromList(things); // This would fail (not because of Java 8 code, but because of non-DSL code that accesses the data) // String csv = things.stream().collect(Collectors.joining(",")); // map().setThingsCSV(csv); System.out.println("Exiting configure()"); } }; ModelMapper modelMapper = new ModelMapper(); modelMapper.addMappings(propertyMap); for (int i=0; i<5; i++) { Foo foo = new Foo(); foo.setThings(Arrays.asList("a"+i, "b"+i, "c"+i)); FooDTO dto = new FooDTO(); modelMapper.map(foo, dto); // The configure method is not re-executed, but the dynamically generated mapper method is. System.out.println(dto.getThingsCSV()); } } public static class Foo { List things; public List getThings() { return things; } public void setThings(List things) { this.things = things; } } public static class FooDTO { String thingsCSV; public String getThingsCSV() { return thingsCSV; } public void setThingsCSV(String thingsCSV) { this.thingsCSV = thingsCSV; } public void setThingsCSVFromList(List things) { setThingsCSV(things.stream().collect(Collectors.joining(","))); } } }
如果你按原样执行它，你会得到：
Entering configure() Exiting configure() a0,b0,c0 a1,b1,c1 a2,b2,c2 a3,b3,c3 a4,b4,c4
因此， configure()只执行一次以记录映射指令，然后生成的映射代码（不是configure()本身） 重放 5次，每次对象映射一次。
如果你在configure()注释掉map().setThingsCSVFromList(things)的行，然后取消注释“这会失败”下面的2行，你得到：
Exception in thread "main" org.modelmapper.ConfigurationException: ModelMapper configuration errors: 1) Invalid source method java.util.stream.Stream.collect(). Ensure that method has zero parameters and does not return void.
简而言之，您无法直接在PropertyMap.configure()执行复杂的自定义逻辑，但您可以调用这样做的方法 。 这是因为框架只需要检测处理纯映射逻辑（即DSL）的字节码部分，它不关心这些方法中发生了什么。
解
（A – legacy，for Java 6/7）严格限制DSL所要求的configure内容。 例如，将您的“特殊需求”（记录，收集逻辑等）移动到DTO本身的专用方法。
在你的情况下，将这种逻辑转移到其他地方可能会有更多的工作，但这个想法就在那里。
请注意，doc表示PropertyMap.configure ，其DSL主要用于Java 6/7，但Java 8和lambdas现在允许优雅的解决方案，其优点是不需要字节码操作魔法。
（B – Java 8）查看其他选项 ，例如Converter 。
这是另一个例子（使用与上面相同的数据类，以及整个类型的Converter ，因为这更适合我的例子，但你可以按属性执行）：
Converter converter = context -> { FooDTO dto = new FooDTO(); dto.setThingsCSV( context.getSource().getThings().stream() .collect(Collectors.joining(","))); return dto; }; ModelMapper modelMapper = new ModelMapper(); modelMapper.createTypeMap(Foo.class, FooDTO.class) .setConverter(converter); Foo foo = new Foo(); foo.setThings(Arrays.asList("a", "b", "c")); FooDTO dto = modelMapper.map(foo, FooDTO.class); System.out.println(dto.getThingsCSV()); // a,b,c