java - 使用 Eclipselink 和 Moxy 将 List 写入文件

Question

我正在使用 Eclipselink JPA 并有一组数据库实体，我想将其写入一个文件，以防我的数据库连接在后续查询中不可用。我想使用 Moxy 简单地编码我的整个结果集，然后在稍后解码该文件，从而重新创建我的原始结果集(这是我的 JPA 实体对象)。因为 Eclipselink 和 Moxy 在某种程度上是集成的，所以我想知道我的 JPA 代码，例如:

public void getDataFROMDATABASE() {

factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME,getProperties());
EntityManager em = factory.createEntityManager();
// Read the existing entries and write to console
TypedQuery<CtoPolicyMatrix> query = em.createQuery("SELECT pm FROM CtoPolicyMatrix pm", CtoPolicyMatrix.class);

List<CtoPolicyMatrix> results = query.getResultList();
**//NOW PRESIST RESULTS to file IN CASE OF DB CONNECTION FAILURE**
    presistMatrixData(results);     
 em.close();
}

public void presistMatrixData(List results){
// Save CtoPolicyMatrix to XML
try {
    jc = JAXBContext.newInstance(CtoPolicyMatrix.class);
    Marshaller marshaller = jc.createMarshaller();
    StringWriter writer = new StringWriter();
    marshaller.marshal(results, writer);
    **???? NOT SURE WHAT TO DO HERE TO WRITE MY COLLECTION TO FILE**
    } catch (JAXBException e) {
        e.printStackTrace();
    }
}

public void retrieveMatrixData(List results){
        // Load CtoPolicyMatrix from XML
**???? REALLY NOT SURE HOW TO RETRIEVE MY JPA ENTITY COLLECTION FROM FILE**
Unmarshaller unmarshaller;
    try {
        unmarshaller = jc.createUnmarshaller();
        StringReader reader = new StringReader(writer.toString());
        List<CtoPolicyMatrix> savedResults = (List<CtoPolicyMatrix>)     
            Unmarshaller.unmarshal(reader);
    } catch (JAXBException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

感谢您提供的任何帮助。

Answer 1

您走在正确的轨道上。为了让事情尽可能简单，我建议创建一个包装器对象来包含您的结果列表，然后编码/取消编码该包装器对象。

包装器对象可能如下所示:

import java.util.List;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class MatrixData {

    private List<CtoPolicyMatrix> data;

    public List<CtoPolicyMatrix> getData() {
        return data;
    }

    public void setData(List<CtoPolicyMatrix> data) {
        this.data = data;
    }

}

您的 persistMatrixData 方法如下所示:

public void persistMatrixData(List results) {
    // Save CtoPolicyMatrix to XML
    try {
        MatrixData data = new MatrixData();
        data.setData(results);

        File file = new File("D:/Temp/MatrixData.xml");

        jc = JAXBContext.newInstance(MatrixData.class);
        Marshaller marshaller = jc.createMarshaller();
        marshaller.marshal(data, file);
    } catch (JAXBException e) {
        e.printStackTrace();
    }
}

然后读回它们:

public void retrieveMatrixData(List results) {
    // Load CtoPolicyMatrix from XML
    Unmarshaller unmarshaller;
    try {
        File file = new File("D:/Temp/MatrixData.xml");

        unmarshaller = jc.createUnmarshaller();
        MatrixData data = (MatrixData) u.unmarshal(file);

        List<CtoPolicyMatrix> savedResults = data.getData();
    } catch (JAXBException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

希望对您有所帮助，

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