hibernate 普通欄位消極式載入無效的解決辦法，hibernate欄位

hibernate 普通欄位消極式載入無效的解決辦法，hibernate欄位

關聯對象的消極式載入就不說了，大家都知道。

關於普通欄位的消極式載入，尤其是lob欄位，若沒有消極式載入，對效能影響極大。然而簡單的使用 @Basic(fetch = FetchType.LAZY) 註解並沒有效果。hibernate對此的解釋是Lazy property loading requires buildtime bytecode instrumentation. If your persistent classes are not enhanced, Hibernate will ignore lazy property settings and return to immediate fetching.

而bytecode instrumentation的介紹可以參考http://www.correlsense.com/blog/java-bytecode-instrumentation-an-introduction/，本文不多作介紹。

正是因為我們的persistent classes沒有使用bytecode instrumentation增強，才導致了普通欄位無法消極式載入。

因此要改寫一下。以下為一個使用了bytecode instrumentation的持久類：

public class PublicSchemeTaskFile  implements java.io.Serializable , FieldHandled  {    // Fields         /** *  */     private static final long serialVersionUID = -8297912895820802249L;     private Integer id;     private PublicTask publicSchemeTask;     private Integer fileType;     private String fileName;     private byte[] content;          private FieldHandler fieldHandler;//用於消極式載入表欄位，關聯對象消極式載入的話無需此技術  @JSON(serialize = false) public FieldHandler getFieldHandler() { return fieldHandler; } public void setFieldHandler(FieldHandler fieldHandler) { this.fieldHandler = fieldHandler; }    // Constructors    /** default constructor */    public PublicSchemeTaskFile() {    }/** minimal constructor */    public PublicSchemeTaskFile(Integer id) {        this.id = id;    }         // Property accessors    @Id         @Column(name="ID", unique=true, nullable=false, precision=22, scale=0)    @GeneratedValue(strategy=GenerationType.SEQUENCE,generator = "PUBLIC_SCHEME_TASK_FILE_SEQ")    public Integer getId() {        return this.id;    }        public void setId(Integer id) {        this.id = id;    }    @JSON(serialize = false)@ManyToOne(fetch=FetchType.LAZY)     @JoinColumn(name="PUBLIC_TASK_ID")    public PublicTask getPublicSchemeTask() {        return this.publicSchemeTask;    }        public void setPublicSchemeTask(PublicTask publicSchemeTask) {        this.publicSchemeTask = publicSchemeTask;    }        @Column(name="FILE_TYPE", precision=22, scale=0)    public Integer getFileType() {        return this.fileType;    }        public void setFileType(Integer fileType) {        this.fileType = fileType;    }        @Column(name="FILE_NAME", length=50)    public String getFileName() {        return this.fileName;    }        public void setFileName(String fileName) {        this.fileName = fileName;    }    @JSON(serialize = false)    @Lob    @Basic(fetch = FetchType.LAZY)     @Column(name="CONTENT")    public byte[] getContent() {     if (fieldHandler != null) {        return (byte[]) fieldHandler.readObject(this, "content", content);     } return null;    }        public void setContent(byte[] content) {        this.content = content;    } @Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + ((id == null) ? 0 : id.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;PublicSchemeTaskFile other = (PublicSchemeTaskFile) obj;if (id == null) {if (other.id != null)return false;} else if (!id.equals(other.id))return false;return true;}   }

關鍵在於FieldHandled介面和lob欄位的getter

hibernate消極式載入問題

Hibernate.initialize(stu.getCourses());
需要在SESSION關閉前。而過濾器實質上也是在SESSION前。

樓主可以再開一個SESSION，然後根據拿到的課程集合查詢課程。
 
hibernate問題消極式載入問題

在web.xml中配置OpenSessionInView過濾器即可解決

<!-- OpenSessionInView 配置，解決Hibernate消極式載入的問題 -->
<filter>
<filter-name>lazyLoadingFilter</filter-name>
<filter-class>org.springframework.orm.hibernate3.support.OpenSessionInViewFilter</filter-class>
</filter>

<filter-mapping>
<filter-name>lazyLoadingFilter</filter-name>
<url-pattern>*.do</url-pattern>
</filter-mapping>