Google Guava單次查詢or批量查詢

引子：

寫代碼中，如果在for中有查詢數據的，一般大多數公司，都要求批量查詢，或者說不讓在for中查詢太多的數據庫。今天簡單說一下

1.需求：

學生和科目。學生和科目是一對的關係。現在要求查詢出全班學生的所有科目的成績。

實體：

student :學生實體

subject：科目實體

<code>public class Student {
    private Long id;
    private String name;
    private Integer age;
    private Integer sex;
    private List<subject> subjectList;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public Integer getSex() {
        return sex;
    } 


    public void setSex(Integer sex) {
        this.sex = sex;
    }

    public List<subject> getSubjectList() {
        return subjectList;
    }

    public void setSubjectList(List<subject> subjectList) {
        this.subjectList = subjectList;
    }
}

public class Subject {
    private Long id;
    private Long studentId;
    private String name;
    private Float age;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public Long getStudentId() {
        return studentId;
    }

    public void setStudentId(Long studentId) {
        this.studentId = studentId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Float getAge() {
        return age;
    }

    public void setAge(Float age) {
        this.age = age; 

    }
}/<subject>/<subject>/<subject>/<code>

<code>/** * 查詢學生信息 * @param studentIds * @return */
private List<student> queryStudent(List<long> studentIds) {
    // 查詢數據庫    
  return new ArrayList<>();
}

/** * 查詢科目信息 * @param subjectIds * @return */
private List<subject> querySubject(List<long> subjectIds) {
    // 查詢數據庫    
  return Lists.newArrayList();
}

/** * 查詢科目信息 * @param subjectId * @return */
private List<subject> querySubject(Long subjectId) {
    // 查詢數據庫    
  return Lists.newArrayList();
}/<subject>/<long>/<subject>/<long>/<student>/<code>

2.不同方案

2.1單次查詢：

<code>/** * 單次查詢 * @param ids * @return */
private List<student> queryStudentAndSubject(List<long> ids){
    List<student> students = queryStudent(ids);
    students.forEach(student->{
        List<subject> subjects = querySubject(student.id);
        student.setSubjectList(subjects);
    });
    return students;
}/<subject>/<student>/<long>/<student>/<code>

2.2批量查詢：

<code>/** * 批量查詢 * @param ids * @return */private List<student> queryBatchStudentAndSubject(List<long> ids){
    List<student> students = queryStudent(ids);
    List<subject> subjects = querySubject(ids);
    Map<long>> studentMap=new HashMap<>();
    subjects.forEach(subject -> {
        Long studentId = subject.getStudentId();
        List<subject> subjectsList = studentMap.get(studentMap);
        if(CollectionUtils.isEmpty(subjectsList)){ 

            subjectsList=new ArrayList<>();
            studentMap.put(studentId,subjectsList);
        }
        subjectsList.add(subject);
    });
    students.forEach(student->{
        List<subject> subjects1 = studentMap.get(student.getId());
        student.setSubjectList(subjects1);
    });
    return students;
}/<subject>/<subject>/<long>/<subject>/<student>/<long>/<student>/<code>

2.3批量查詢：使用Guava寫法1

<code>/** * 批量查詢 * @param ids * @return */
private List<student> queryBatchStudentAndSubject2(List<long> ids){
    List<student> students = queryStudent(ids);
    List<subject> subjects = querySubject(ids);
    ArrayListMultimap<long> map=new ArrayListMultimap<>();
    subjects.forEach(subject -> {
        map.put(subject.getStudentId(),subject);
    });
    students.forEach(student->{
        List<subject> subjectList = map.get(student.getId());
        student.setSubjectList(subjectList);
    });
    return students;
}/<subject>/<long>/<subject>/<student>/<long>/<student>/<code>

2.4批量查詢：使用Guava寫法2

<code>/** * 批量查詢 * @param ids * @return */
private List<student> queryBatchStudentAndSubject3(List<long> ids){
    List<student> students = queryStudent(ids);
    List<subject> subjects = querySubject(ids);
    Map<long>> map = Multimaps.index(subjects, Subject::getStudentId).asMap();
    students.forEach(student->{
        Collection<subject> subjectsList = map.get(student.getId());
        student.setSubjectList(new ArrayList<>(subjectsList));
    });
    return students;
}/<subject>/<long>/<subject>/<student>/<long>/<student>/<code>

2.5對比

2.1和2.2

<code>ArrayListMultimap<long> map=new ArrayListMultimap<>(); 

        subjects.forEach(subject -> {
            map.put(subject.getStudentId(),subject);
        });
        students.forEach(student->{
            List<subject> subjectList = map.get(student.getId());
            student.setSubjectList(subjectList);
        });/<subject>/<long>/<code>

<code>Map<long>> studentMap=new HashMap<>();
subjects.forEach(subject -> {
    Long studentId = subject.getStudentId();
    List<subject> subjectsList = studentMap.get(studentMap);
    if(CollectionUtils.isEmpty(subjectsList)){
        subjectsList=new ArrayList<>();
        studentMap.put(studentId,subjectsList);
    }
    subjectsList.add(subject);
});/<subject>/<long>/<code>

2.1和2.3

<code>Map<long>> map = Multimaps.index(subjects, Subject::getStudentId).asMap();
students.forEach(student->{
    Collection<subject> subjectsList = map.get(student.getId());
    student.setSubjectList(new ArrayList<>(subjectsList));
});/<subject>/<long>/<code>

<code>Map<long>> studentMap=new HashMap<>();
subjects.forEach(subject -> {
    Long studentId = subject.getStudentId();
    List<subject> subjectsList = studentMap.get(studentMap);
    if(CollectionUtils.isEmpty(subjectsList)){
        subjectsList=new ArrayList<>();
        studentMap.put(studentId,subjectsList);
    }
    subjectsList.add(subject);
});
students.forEach(student->{
    List<subject> subjects1 = studentMap.get(student.getId());
    student.setSubjectList(subjects1);
});/<subject>/<subject>/<long>/<code>

可以看出代碼越來越精簡。

這就是為什麼強烈推薦Java程序員使用Google Guava編程。

上面只是其中的點點。Guava還有其他優雅的寫法。以後再說。

http://code.google.com/p/guava-libraries/wiki/GuavaExplained

最後附上Multimap提供了豐富的實現（從網上找了一個圖）

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關鍵字: Google 單次 批量