資料對映器模式（Data Mapper Pattern）

資料庫 · 發表 2019-03-21 09:40:00

摘要：該模式屬於資料來源架構模式目錄，此目錄屬於企業應用程式體系結構模式。目的一個對映器層，在物件和資料庫之間移動資料，同時保持它們彼此和對映器本身獨立。對映器（Mapper ）是指在兩個獨立物件之間建立通訊的物件。適用性在以下任何情況下使用Data Ma...

該模式屬於 資料來源架構模式 目錄，此目錄屬於 企業應用程式體系結構模式 。

目的

一個對映器層，在物件和資料庫之間移動資料，同時保持它們彼此和對映器本身獨立。對映器（Mapper ）是指在兩個獨立物件之間建立通訊的物件。

適用性

在以下任何情況下使用Data Mapper

當您想要從DB訪問層分離資料物件時

當您想要編寫多個數據檢索/永續性實現時

說明

Data Mapper是一個軟體層，用於將記憶體中物件與資料庫分開。

它的職責是在兩者之間傳輸資料，並將它們彼此隔離。

使用Data Mapper，即使存在資料庫，記憶體中的物件也不需要知道; 他們不需要SQL介面程式碼，當然也不需要了解資料庫模式。

如何運作

域和資料來源之間的分離是Data Mapper的主要功能，但要實現這一點，需要處理大量的細節。

一個簡單的案例要有一個 Person 和 Person Mapper 類。若要從資料庫載入人員，客戶端將在mapper上呼叫find方法。對映器使用Identity Map模式來檢視此人是否已載入; 如果沒有，它載入它。

示例程式碼

讓我們為 StudentMapper的 示例程式碼建立一個類圖，以演示這種模式。

第1步：建立Student域類。

<b>public</b> <b>final</b> <b>class</b> Student implements Serializable {

<b>private</b> <b>static</b> <b>final</b> <b>long</b> serialVersionUID = 1L;

<b>private</b> <b>int</b> studentId;
<b>private</b> String name;
<b>private</b> <b>char</b> grade;


<font><i>/**
* Use this constructor to create a Student with all details
*
* @param studentId as unique student id
* @param name as student name
* @param grade as respective grade of student
*/</i></font><font>
<b>public</b> Student(<b>final</b> <b>int</b> studentId, <b>final</b> String name, <b>final</b> <b>char</b> grade) {
<b>this</b>.studentId = studentId;
<b>this</b>.name = name;
<b>this</b>.grade = grade;
}

</font><font><i>/**
*
* @return the student id
*/</i></font><font>
<b>public</b> <b>int</b> getStudentId() {
<b>return</b> studentId;
}

</font><font><i>/**
*
* @param studentId as unique student id
*/</i></font><font>
<b>public</b> <b>void</b> setStudentId(<b>final</b> <b>int</b> studentId) {
<b>this</b>.studentId = studentId;
}

</font><font><i>/**
*
* @return name of student
*/</i></font><font>
<b>public</b> String getName() {
<b>return</b> name;
}

</font><font><i>/**
*
* @param name as 'name' of student
*/</i></font><font>
<b>public</b> <b>void</b> setName(<b>final</b> String name) {
<b>this</b>.name = name;
}

</font><font><i>/**
*
* @return grade of student
*/</i></font><font>
<b>public</b> <b>char</b> getGrade() {
<b>return</b> grade;
}

</font><font><i>/**
*
* @param grade as 'grade of student'
*/</i></font><font>
<b>public</b> <b>void</b> setGrade(<b>final</b> <b>char</b> grade) {
<b>this</b>.grade = grade;
}

</font><font><i>/**
 *
 */</i></font><font>
@Override
<b>public</b> <b>boolean</b> equals(<b>final</b> Object inputObject) {

<b>boolean</b> isEqual = false;

</font><font><i>/* Check if both objects are same */</i></font><font>
<b>if</b> (<b>this</b> == inputObject) {

isEqual = <b>true</b>;
} <b>else</b> <b>if</b> (inputObject != <b>null</b> && getClass() == inputObject.getClass()) {

<b>final</b> Student inputStudent = (Student) inputObject;

</font><font><i>/* If student id matched */</i></font><font>
<b>if</b> (<b>this</b>.getStudentId() == inputStudent.getStudentId()) {

isEqual = <b>true</b>;
}
}

<b>return</b> isEqual;
}

</font><font><i>/**
 *
 */</i></font><font>
@Override
<b>public</b> <b>int</b> hashCode() {

</font><font><i>/* Student id is assumed to be unique */</i></font><font>
<b>return</b> <b>this</b>.getStudentId();
}

</font><font><i>/**
 *
 */</i></font><font>
@Override
<b>public</b> String toString() {
<b>return</b> </font><font>"Student [studentId="</font><font> + studentId + </font><font>", name="</font><font> + name + </font><font>", grade="</font><font> + grade + </font><font>"]"</font><font>;
}
}
</font>

第2步：讓我們使用Runtime Exception建立，以避免依賴於實現異常。這有助於解耦。

<b>public</b> <b>final</b> <b>class</b> DataMapperException <b>extends</b> RuntimeException {

<b>private</b> <b>static</b> <b>final</b> <b>long</b> serialVersionUID = 1L;

<font><i>/**
* Constructs a new runtime exception with the specified detail message. The cause is not
* initialized, and may subsequently be initialized by a call to {@link #initCause}.
*
* @param message the detail message. The detail message is saved for later retrieval by the
*{@link #getMessage()} method.
*/</i></font><font>
<b>public</b> DataMapperException(<b>final</b> String message) {
<b>super</b>(message);
}
}
</font>

第3步：建立 StudentDataMapper 介面 - 介面列出了所有可能的學生對映器的可能行為。

<b>public</b> <b>interface</b> StudentDataMapper {

Optional<Student> find(<b>int</b> studentId);

<b>void</b> insert(Student student) throws DataMapperException;

<b>void</b> update(Student student) throws DataMapperException;

<b>void</b> delete(Student student) throws DataMapperException;
}

第4步：讓我們實現上面的介面 - 對學生資料執行動作。這個實現是在記憶體中，你可以在這裡使用資料庫連線。

<b>public</b> <b>final</b> <b>class</b> StudentDataMapperImpl implements StudentDataMapper {

<font><i>/* Note: Normally this would be in the form of an actual database */</i></font><font>
<b>private</b> List<Student> students = <b>new</b> ArrayList<>();

@Override
<b>public</b> Optional<Student> find(<b>int</b> studentId) {

</font><font><i>/* Compare with existing students */</i></font><font>
<b>for</b> (<b>final</b> Student student : <b>this</b>.getStudents()) {

</font><font><i>/* Check if student is found */</i></font><font>
<b>if</b> (student.getStudentId() == studentId) {

<b>return</b> Optional.of(student);
}
}

</font><font><i>/* Return empty value */</i></font><font>
<b>return</b> Optional.empty();
}

@Override
<b>public</b> <b>void</b> update(Student studentToBeUpdated) throws DataMapperException {


</font><font><i>/* Check with existing students */</i></font><font>
<b>if</b> (<b>this</b>.getStudents().contains(studentToBeUpdated)) {

</font><font><i>/* Get the index of student in list */</i></font><font>
<b>final</b> <b>int</b> index = <b>this</b>.getStudents().indexOf(studentToBeUpdated);

</font><font><i>/* Update the student in list */</i></font><font>
<b>this</b>.getStudents().set(index, studentToBeUpdated);

} <b>else</b> {

</font><font><i>/* Throw user error after wrapping in a runtime exception */</i></font><font>
<b>throw</b> <b>new</b> DataMapperException(</font><font>"Student ["</font><font> + studentToBeUpdated.getName() + </font><font>"] is not found"</font><font>);
}
}

@Override
<b>public</b> <b>void</b> insert(Student studentToBeInserted) throws DataMapperException {

</font><font><i>/* Check with existing students */</i></font><font>
<b>if</b> (!<b>this</b>.getStudents().contains(studentToBeInserted)) {

</font><font><i>/* Add student in list */</i></font><font>
<b>this</b>.getStudents().add(studentToBeInserted);

} <b>else</b> {

</font><font><i>/* Throw user error after wrapping in a runtime exception */</i></font><font>
<b>throw</b> <b>new</b> DataMapperException(</font><font>"Student already ["</font><font> + studentToBeInserted.getName() + </font><font>"] exists"</font><font>);
}
}

@Override
<b>public</b> <b>void</b> delete(Student studentToBeDeleted) throws DataMapperException {

</font><font><i>/* Check with existing students */</i></font><font>
<b>if</b> (<b>this</b>.getStudents().contains(studentToBeDeleted)) {

</font><font><i>/* Delete the student from list */</i></font><font>
<b>this</b>.getStudents().remove(studentToBeDeleted);

} <b>else</b> {

</font><font><i>/* Throw user error after wrapping in a runtime exception */</i></font><font>
<b>throw</b> <b>new</b> DataMapperException(</font><font>"Student ["</font><font> + studentToBeDeleted.getName() + </font><font>"] is not found"</font><font>);
}
}

<b>public</b> List<Student> getStudents() {
<b>return</b> <b>this</b>.students;
}
}
</font>

第5步：讓我們測試一下這種模式。下面的Client類演示了基本的CRUD操作：建立，讀取，更新和刪除。

<b>public</b> <b>final</b> <b>class</b> Client {

<b>private</b> <b>static</b> Logger log = Logger.getLogger(App.<b>class</b>);

<font><i>/**
* Program entry point.
* 
* @param args command line args.
*/</i></font><font>
<b>public</b> <b>static</b> <b>void</b> main(<b>final</b> String... args) {

</font><font><i>/* Create new data mapper for type 'first' */</i></font><font>
<b>final</b> StudentDataMapper mapper = <b>new</b> StudentDataMapperImpl();

</font><font><i>/* Create new student */</i></font><font>
Student student = <b>new</b> Student(1, </font><font>"Adam"</font><font>, 'A');

</font><font><i>/* Add student in respectibe store */</i></font><font>
mapper.insert(student);

log.debug(</font><font>"App.main(), student : "</font><font> + student + </font><font>", is inserted"</font><font>);

</font><font><i>/* Find this student */</i></font><font>
<b>final</b> Optional<Student> studentToBeFound = mapper.find(student.getStudentId());

log.debug(</font><font>"App.main(), student : "</font><font> + studentToBeFound + </font><font>", is searched"</font><font>);

</font><font><i>/* Update existing student object */</i></font><font>
student = <b>new</b> Student(student.getStudentId(), </font><font>"AdamUpdated"</font><font>, 'A');

</font><font><i>/* Update student in respectibe db */</i></font><font>
mapper.update(student);

log.debug(</font><font>"App.main(), student : "</font><font> + student + </font><font>", is updated"</font><font>);
log.debug(</font><font>"App.main(), student : "</font><font> + student + </font><font>", is going to be deleted"</font><font>);

</font><font><i>/* Delete student in db */</i></font><font>
mapper.delete(student);
}

}
</font>

資料對映器模式（Data Mapper Pattern）

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