Page
接口表示一个分页的结果,其中包含当前页的结果、总页数、总元素数以及分页和排序元数据。
PagedListHolder
类是一个可变的列表持有者,支持分页和排序。它提供了诸如查找指定页面的元素、获取总页数以及更改页大小等操作。
总之,本文档介绍了 Pageable
和 Page
接口以及 PagedListHolder
类,这些工具可以简化对分页和排序操作结果的处理。
Object Mapping Fundamentals
本节涵盖了 Spring Data 对象映射、对象创建、字段和属性访问、可变性和不可变性的基础知识。请注意,此部分只适用于未使用基础数据存储的对象映射(如 JPA)的 Spring Data 模块。还要务必查阅特定于存储的部分,了解特定于存储的对象映射,如索引、自定义列或字段名称等。
This section covers the fundamentals of Spring Data object mapping, object creation, field and property access, mutability and immutability. Note, that this section only applies to Spring Data modules that do not use the object mapping of the underlying data store (like JPA). Also be sure to consult the store-specific sections for store-specific object mapping, like indexes, customizing column or field names or the like.
Spring Data 对象映射的核心职责是创建域对象实例并将存储本地数据结构映射到这些实例上。这意味着我们需要两个基本步骤:
Core responsibility of the Spring Data object mapping is to create instances of domain objects and map the store-native data structures onto those. This means we need two fundamental steps:
-
Instance creation by using one of the constructors exposed.
-
Instance population to materialize all exposed properties.
Object creation
Spring Data 自动尝试检测一个持久实体的构造函数,以便用于实现该类型的对象。解决算法的工作方式如下:
Spring Data automatically tries to detect a persistent entity’s constructor to be used to materialize objects of that type. The resolution algorithm works as follows:
-
If there is a single static factory method annotated with
@PersistenceCreator
then it is used. -
If there is a single constructor, it is used.
-
If there are multiple constructors and exactly one is annotated with
@PersistenceCreator
, it is used. -
If the type is a Java
Record
the canonical constructor is used. -
If there’s a no-argument constructor, it is used. Other constructors will be ignored.
值解析假定构造函数/工厂方法的参数名称与实体的属性名称匹配,即解析将执行得好像要填充该属性一样,包括映射中的所有定制(不同的数据存储列或字段名称等)。这也要求在类文件或构造函数上有 @ConstructorProperties
注释中提供参数名称信息。
The value resolution assumes constructor/factory method argument names to match the property names of the entity, i.e. the resolution will be performed as if the property was to be populated, including all customizations in mapping (different datastore column or field name etc.).
This also requires either parameter names information available in the class file or an @ConstructorProperties
annotation being present on the constructor.
可以通过使用 Spring Framework 的 @Value
值注释来使用特定于存储的 SpEL 表达式来定制值解析。请查阅有关具体存储的映射的部分以获取更多详细信息。
The value resolution can be customized by using Spring Framework’s @Value
value annotation using a store-specific SpEL expression.
Please consult the section on store specific mappings for further details.
为了避免反射时间开销,Spring Data 对象创建在默认情况下使用在运行时生成的工厂类,该类将直接调用领域类的构造函数。对于以下示例类型:
To avoid the overhead of reflection, Spring Data object creation uses a factory class generated at runtime by default, which will call the domain classes constructor directly. I.e. for this example type:
class Person {
Person(String firstname, String lastname) { … }
}
我们将在运行时为这个类创建一个语义上等价的工厂类:
we will create a factory class semantically equivalent to this one at runtime:
class PersonObjectInstantiator implements ObjectInstantiator {
Object newInstance(Object... args) {
return new Person((String) args[0], (String) args[1]);
}
}
这给了我们超过反射 10% 的迂回式性能提升。为了使领域类适用于这种优化,它需要遵守一组约束:
This gives us a roundabout 10% performance boost over reflection. For the domain class to be eligible for such optimization, it needs to adhere to a set of constraints:
-
it must not be a private class
-
it must not be a non-static inner class
-
it must not be a CGLib proxy class
-
the constructor to be used by Spring Data must not be private
如果符合上述任何一个标准,Spring Data 将回退到通过反射进行实体实例化。
If any of these criteria match, Spring Data will fall back to entity instantiation via reflection.
Property population
在创建了该实体的一个实例之后,Spring Data 填充该类的所有剩余的持久属性。除非已通过实体的构造函数填充(即通过其构造函数参数列表消耗),否则将首先填充标识符属性,以允许解析循环对象引用。在此之后,所有尚未被构造函数填充的非瞬态属性都会在实体实例上设置。为此,我们使用以下算法:
Once an instance of the entity has been created, Spring Data populates all remaining persistent properties of that class. Unless already populated by the entity’s constructor (i.e. consumed through its constructor argument list), the identifier property will be populated first to allow the resolution of cyclic object references. After that, all non-transient properties that have not already been populated by the constructor are set on the entity instance. For that we use the following algorithm:
-
If the property is immutable but exposes a
with…
method (see below), we use thewith…
method to create a new entity instance with the new property value. -
If property access (i.e. access through getters and setters) is defined, we’re invoking the setter method.
-
If the property is mutable we set the field directly.
-
If the property is immutable we’re using the constructor to be used by persistence operations (see mapping.object-creation) to create a copy of the instance.
-
By default, we set the field value directly.
类似于我们在对象构造中的优化,映射.object-creation.details,我们还使用 Spring Data 运行时生成的访问器类与实体实例进行交互。
Similarly to our mapping.object-creation.details we also use Spring Data runtime generated accessor classes to interact with the entity instance.
class Person {
private final Long id;
private String firstname;
private @AccessType(Type.PROPERTY) String lastname;
Person() {
this.id = null;
}
Person(Long id, String firstname, String lastname) {
// Field assignments
}
Person withId(Long id) {
return new Person(id, this.firstname, this.lastame);
}
void setLastname(String lastname) {
this.lastname = lastname;
}
}
class PersonPropertyAccessor implements PersistentPropertyAccessor {
private static final MethodHandle firstname; 2
private Person person; 1
public void setProperty(PersistentProperty property, Object value) {
String name = property.getName();
if ("firstname".equals(name)) {
firstname.invoke(person, (String) value); 2
} else if ("id".equals(name)) {
this.person = person.withId((Long) value); 3
} else if ("lastname".equals(name)) {
this.person.setLastname((String) value); 4
}
}
}
1 | PropertyAccessor’s hold a mutable instance of the underlying object. This is, to enable mutations of otherwise immutable properties. |
2 | By default, Spring Data uses field-access to read and write property values. As per visibility rules of private fields, MethodHandles are used to interact with fields. |
3 | The class exposes a withId(…) method that’s used to set the identifier, e.g. when an instance is inserted into the datastore and an identifier has been generated. Calling withId(…) creates a new Person object. All subsequent mutations will take place in the new instance leaving the previous untouched. |
4 | Using property-access allows direct method invocations without using MethodHandles . |
这给了我们超过反射 25% 的迂回式性能提升。为了使领域类适用于这种优化,它需要遵守一组约束:
This gives us a roundabout 25% performance boost over reflection. For the domain class to be eligible for such optimization, it needs to adhere to a set of constraints:
-
Types must not reside in the default or under the
java
package. -
Types and their constructors must be
public
-
Types that are inner classes must be
static
. -
The used Java Runtime must allow for declaring classes in the originating
ClassLoader
. Java 9 and newer impose certain limitations.
在默认情况下,Spring Data 尝试使用生成的属性访问器,如果检测到限制,则回退到基于反射的属性访问器。
By default, Spring Data attempts to use generated property accessors and falls back to reflection-based ones if a limitation is detected.
让我们看看以下实体:
Let’s have a look at the following entity:
class Person {
private final @Id Long id; 1
private final String firstname, lastname; 2
private final LocalDate birthday;
private final int age; 3
private String comment; 4
private @AccessType(Type.PROPERTY) String remarks; 5
static Person of(String firstname, String lastname, LocalDate birthday) { 6
return new Person(null, firstname, lastname, birthday,
Period.between(birthday, LocalDate.now()).getYears());
}
Person(Long id, String firstname, String lastname, LocalDate birthday, int age) { 6
this.id = id;
this.firstname = firstname;
this.lastname = lastname;
this.birthday = birthday;
this.age = age;
}
Person withId(Long id) { 1
return new Person(id, this.firstname, this.lastname, this.birthday, this.age);
}
void setRemarks(String remarks) { 5
this.remarks = remarks;
}
}
1 | The identifier property is final but set to null in the constructor.
The class exposes a withId(…) method that’s used to set the identifier, e.g. when an instance is inserted into the datastore and an identifier has been generated.
The original Person instance stays unchanged as a new one is created.
The same pattern is usually applied for other properties that are store managed but might have to be changed for persistence operations.
The wither method is optional as the persistence constructor (see 6) is effectively a copy constructor and setting the property will be translated into creating a fresh instance with the new identifier value applied. |
2 | The firstname and lastname properties are ordinary immutable properties potentially exposed through getters. |
3 | The age property is an immutable but derived one from the birthday property.
With the design shown, the database value will trump the defaulting as Spring Data uses the only declared constructor.
Even if the intent is that the calculation should be preferred, it’s important that this constructor also takes age as parameter (to potentially ignore it) as otherwise the property population step will attempt to set the age field and fail due to it being immutable and no with… method being present. |
4 | The comment property is mutable and is populated by setting its field directly. |
5 | The remarks property is mutable and is populated by invoking the setter method. |
6 | The class exposes a factory method and a constructor for object creation.
The core idea here is to use factory methods instead of additional constructors to avoid the need for constructor disambiguation through @PersistenceCreator .
Instead, defaulting of properties is handled within the factory method.
If you want Spring Data to use the factory method for object instantiation, annotate it with @PersistenceCreator . |
General recommendations
-
Try to stick to immutable objects — Immutable objects are straightforward to create as materializing an object is then a matter of calling its constructor only. Also, this avoids your domain objects to be littered with setter methods that allow client code to manipulate the objects state. If you need those, prefer to make them package protected so that they can only be invoked by a limited amount of co-located types. Constructor-only materialization is up to 30% faster than properties population.
-
Provide an all-args constructor — Even if you cannot or don’t want to model your entities as immutable values, there’s still value in providing a constructor that takes all properties of the entity as arguments, including the mutable ones, as this allows the object mapping to skip the property population for optimal performance.
-
Use factory methods instead of overloaded constructors to avoid `@PersistenceCreator` — With an all-argument constructor needed for optimal performance, we usually want to expose more application use case specific constructors that omit things like auto-generated identifiers etc. It’s an established pattern to rather use static factory methods to expose these variants of the all-args constructor.
-
Make sure you adhere to the constraints that allow the generated instantiator and property accessor classes to be used —
-
For identifiers to be generated, still use a final field in combination with an all-arguments persistence constructor (preferred) or a
with…
method — -
Use Lombok to avoid boilerplate code — As persistence operations usually require a constructor taking all arguments, their declaration becomes a tedious repetition of boilerplate parameter to field assignments that can best be avoided by using Lombok’s
@AllArgsConstructor
.
Overriding Properties
Java 允许对领域类进行灵活的设计,其中子类可以定义在超类中已经使用相同名称声明的属性。考虑以下示例:
Java’s allows a flexible design of domain classes where a subclass could define a property that is already declared with the same name in its superclass. Consider the following example:
public class SuperType {
private CharSequence field;
public SuperType(CharSequence field) {
this.field = field;
}
public CharSequence getField() {
return this.field;
}
public void setField(CharSequence field) {
this.field = field;
}
}
public class SubType extends SuperType {
private String field;
public SubType(String field) {
super(field);
this.field = field;
}
@Override
public String getField() {
return this.field;
}
public void setField(String field) {
this.field = field;
// optional
super.setField(field);
}
}
这两个类都使用可分配的类型定义了 field
。但是,SubType
隐藏了 SuperType.field
。根据类设计,使用构造函数可能是设置 SuperType.field
的唯一默认方式。或者,在设置器中调用 super.setField(…)
可以设置 SuperType
中的 field
。由于这些属性共享相同名称,但可能表示两个不同的值,所有这些机制在某种程度上都会产生冲突。如果类型不可分配,Spring Data 将跳过超类型属性。也就是说,被重写的属性的类型必须可以分配给其超类型属性类型才能被注册为覆盖项,否则超类型属性被视为瞬态。我们通常建议使用不同的属性名称。
Both classes define a field
using assignable types. SubType
however shadows SuperType.field
.
Depending on the class design, using the constructor could be the only default approach to set SuperType.field
.
Alternatively, calling super.setField(…)
in the setter could set the field
in SuperType
.
All these mechanisms create conflicts to some degree because the properties share the same name yet might represent two distinct values.
Spring Data skips super-type properties if types are not assignable.
That is, the type of the overridden property must be assignable to its super-type property type to be registered as override, otherwise the super-type property is considered transient.
We generally recommend using distinct property names.
Spring Data 模块普遍支持包含不同值的重写属性。从编程模型的角度来看,有几件事需要考虑:
Spring Data modules generally support overridden properties holding different values. From a programming model perspective there are a few things to consider:
-
Which property should be persisted (default to all declared properties)? You can exclude properties by annotating these with
@Transient
. -
How to represent properties in your data store? Using the same field/column name for different values typically leads to corrupt data so you should annotate least one of the properties using an explicit field/column name.
-
Using
@AccessType(PROPERTY)
cannot be used as the super-property cannot be generally set without making any further assumptions of the setter implementation.
Kotlin support
Spring Data 适应 Kotlin 的特殊性,允许创建和更改对象。
Spring Data adapts specifics of Kotlin to allow object creation and mutation.
Kotlin object creation
支持实例化 Kotlin 类,所有类在默认情况下都是不可变的,需要显式属性声明才能定义可变属性。
Kotlin classes are supported to be instantiated, all classes are immutable by default and require explicit property declarations to define mutable properties.
Spring Data 自动尝试检测一个持久实体的构造函数,以便用于实现该类型的对象。解决算法的工作方式如下:
Spring Data automatically tries to detect a persistent entity’s constructor to be used to materialize objects of that type. The resolution algorithm works as follows:
-
If there is a constructor that is annotated with
@PersistenceCreator
, it is used. -
If the type is a mapping.kotlin the primary constructor is used.
-
If there is a single static factory method annotated with
@PersistenceCreator
then it is used. -
If there is a single constructor, it is used.
-
If there are multiple constructors and exactly one is annotated with
@PersistenceCreator
, it is used. -
If the type is a Java
Record
the canonical constructor is used. -
If there’s a no-argument constructor, it is used. Other constructors will be ignored.
考虑以下 data
类 Person
:
Consider the following data
class Person
:
data class Person(val id: String, val name: String)
上述类编译为具有显式构造函数的典型类。我们可以通过添加另一个构造函数并用 @PersistenceCreator
注释它来定制此类,以指示构造函数偏好:
The class above compiles to a typical class with an explicit constructor.We can customize this class by adding another constructor and annotate it with @PersistenceCreator
to indicate a constructor preference:
data class Person(var id: String, val name: String) {
@PersistenceCreator
constructor(id: String) : this(id, "unknown")
}
Kotlin 支持参数可选择性,允许在不提供参数时使用默认值。当 Spring Data 检测到带有参数默认值的构造函数时,它会让这些参数保持缺失状态(或仅返回 null
),以便 Kotlin 可以应用参数默认值。考虑以下对 name
应用参数默认值的类
Kotlin supports parameter optionality by allowing default values to be used if a parameter is not provided.
When Spring Data detects a constructor with parameter defaulting, then it leaves these parameters absent if the data store does not provide a value (or simply returns null
) so Kotlin can apply parameter defaulting.Consider the following class that applies parameter defaulting for name
data class Person(var id: String, val name: String = "unknown")
每次当 name
参数既不属于结果的一部分或其值为 null
,则 name
将默认为 unknown
。
Every time the name
parameter is either not part of the result or its value is null
, then the name
defaults to unknown
.
Property population of Kotlin data classes
在 Kotlin 中,所有类默认情况下是不可变的,需要显式声明属性来定义可变属性。考虑下面的 data
类 Person
:
In Kotlin, all classes are immutable by default and require explicit property declarations to define mutable properties.
Consider the following data
class Person
:
data class Person(val id: String, val name: String)
这个类实际上是不可变的。它允许创建新的实例,因为 Kotlin 生成了一个 copy(…)
方法,该方法创建新的对象实例,复制所有属性值(从现有对象)并将作为参数提供给该方法的属性值应用进来。
This class is effectively immutable.
It allows creating new instances as Kotlin generates a copy(…)
method that creates new object instances copying all property values from the existing object and applying property values provided as arguments to the method.
Kotlin Overriding Properties
Kotlin 允许声明 `` 来更改子类中的属性。
Kotlin allows declaring property overrides to alter properties in subclasses.
open class SuperType(open var field: Int)
class SubType(override var field: Int = 1) :
SuperType(field) {
}
这种安排使两个名称为 field
的属性得以呈现。Kotlin 中为每个类中每个属性生成了属性访问器(getter 和 setter)。实际上,该代码如下所示:
Such an arrangement renders two properties with the name field
.
Kotlin generates property accessors (getters and setters) for each property in each class.
Effectively, the code looks like as follows:
public class SuperType {
private int field;
public SuperType(int field) {
this.field = field;
}
public int getField() {
return this.field;
}
public void setField(int field) {
this.field = field;
}
}
public final class SubType extends SuperType {
private int field;
public SubType(int field) {
super(field);
this.field = field;
}
public int getField() {
return this.field;
}
public void setField(int field) {
this.field = field;
}
}
SubType
上的 getter 和 setter 仅设置 SubType.field
而不设置 SuperType.field
。在这样的安排中,使用构造函数是设置 SuperType.field
的唯一默认方法。向 SubType
中添加一个方法以通过 this.SuperType.field = …
来设置 SuperType.field
是可能的,但是不属于受支持的约定。属性覆盖在某种程度上创建了冲突,因为这些属性共享相同名称,但可能表示两个不同的值。我们通常建议使用不同的属性名称。
Getters and setters on SubType
set only SubType.field
and not SuperType.field
.
In such an arrangement, using the constructor is the only default approach to set SuperType.field
.
Adding a method to SubType
to set SuperType.field
via this.SuperType.field = …
is possible but falls outside of supported conventions.
Property overrides create conflicts to some degree because the properties share the same name yet might represent two distinct values.
We generally recommend using distinct property names.
Spring Data 模块普遍支持包含不同值的重写属性。从编程模型的角度来看,有几件事需要考虑:
Spring Data modules generally support overridden properties holding different values. From a programming model perspective there are a few things to consider:
-
Which property should be persisted (default to all declared properties)? You can exclude properties by annotating these with
@Transient
. -
How to represent properties in your data store? Using the same field/column name for different values typically leads to corrupt data so you should annotate least one of the properties using an explicit field/column name.
-
Using
@AccessType(PROPERTY)
cannot be used as the super-property cannot be set.
Kotlin Value Classes
Kotlin 值类被设计用于更具表现力的领域模型来使基础概念明确化。Spring Data 可以读取和写入使用值类定义属性的类型。
Kotlin Value Classes are designed for a more expressive domain model to make underlying concepts explicit. Spring Data can read and write types that define properties using Value Classes.
考虑以下领域模型:
Consider the following domain model:
@JvmInline
value class EmailAddress(val theAddress: String) 1
data class Contact(val id: String, val name:String, val emailAddress: EmailAddress) 2
1 | A simple value class with a non-nullable value type. |
2 | Data class defining a property using the EmailAddress value class. |
在已编译类中,使用非基本值类型而非空属性会展平为该值类型。可空基本值类型或可空值中值类型将用包装器类型表示,并影响在数据库中如何表示值类型。 |
Non-nullable properties using non-primitive value types are flattened in the compiled class to the value type. Nullable primitive value types or nullable value-in-value types are represented with their wrapper type and that affects how value types are represented in the database. |