http://blog.csdn.net/dys1990/article/details/38728175

http://stackoverflow.com/questions/2605477/spring-hibernate-blob-lazy-loading

關於普通欄位的延遲載入，尤其是lob欄位，若沒有延遲載入，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.

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延遲載入。

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

關鍵在於FieldHandled介面和lob欄位的getter

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;//用於延遲載入表字段，關聯物件延遲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;
    }
 

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((id == null) ? 0 : id.hashCode());
		return result;
	}

	@Override
	public 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;
	}
   

}
 

但是，僅僅是這樣，對導致需要用這個lazy fetch的欄位時，無法獲取到它的value,一直處於null 的狀態，這時候就需要用FieldHandler來處理了。

如下：

http://stackoverflow.com/questions/7902250/jpa-lazy-at-simple-byte-field-level

I've cheched it on Hibernate v.4.3.5 and JPA v.1.5.0, PostgreSQL 9.3. Worked like a charm. Example：

public class Attachment implements FieldHandled{
    @Transient
    private FieldHandler fieldHandler;
...
...
    @Lob
    @Column(name=CONTENT, nullable=false)
    @Basic(fetch = FetchType.LAZY, optional = false)
    private byte[] content;

...
...
    public byte[] getContent() {
    if(fieldHandler!=null){
        return (byte[])fieldHandler.readObject(this, "content", content);
    }
    return content;
    }

    public void setContent(byte[] content) {
    if(fieldHandler!=null){
        fieldHandler.writeObject(this, "content", this.content, content);
        return;
    }
    this.content = content;
    }

}