hibernate 普通字段延迟加载无效的解决办法

关联对象的延迟加载就不说了，大家都知道。

关于普通字段的延迟加载，尤其是lob字段，若没有延迟加载，对性能影响极大。然而简单的使用 @Basic(fetch = FetchType.LAZY) 注解并没有效果。hibernate对此的解释是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的介绍可以参考http://www.correlsense.com/blog/java-bytecode-instrumentation-an-introduction/，本文不多作介绍。

正是因为我们的persistent classes没有使用bytecode instrumentation增强，才导致了普通字段无法延迟加载。

因此要改写一下。以下为一个使用了bytecode instrumentation的持久类：

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;//用于延迟加载表字段，关联对象延迟加载的话无需此技术

@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;
}

}

关键在于FieldHandled接口和lob字段的getter