使用示例的 Hibernate Session get() 與 load() 差異

Hibernate Session 提供了不同的方法來從資料庫中獲取資料。其中兩個方法是 – get() 和 load()。對於這兩個方法，還有許多重載方法，我們可以根據不同情況使用。乍一看，get() 和 load() 看起來很相似，因為它們都從資料庫中獲取資料，但是它們之間還是有一些區別，讓我們用一個簡單的例子來看看它們。

package com.journaldev.hibernate.main;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;

import com.journaldev.hibernate.model.Employee;
import com.journaldev.hibernate.util.HibernateUtil;

public class HibernateGetVsLoad {

	public static void main(String[] args) {
		
		//準備工作
		SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
		Session session = sessionFactory.openSession();
		Transaction tx = session.beginTransaction();
		
		//get() 範例
		Employee emp = (Employee) session.get(Employee.class, new Long(2));
		System.out.println("Employee get called");
		System.out.println("Employee ID= "+emp.getId());
		System.out.println("Employee Get Details:: "+emp+"\n");
		
		//load() 範例
		Employee emp1 = (Employee) session.load(Employee.class, new Long(1));
		System.out.println("Employee load called");
		System.out.println("Employee ID= "+emp1.getId());
		System.out.println("Employee load Details:: "+emp1+"\n");
		
		//關閉資源
		tx.commit();
		sessionFactory.close();
	}
}

當我執行上面的程式碼時，它產生以下輸出。

Hibernate: select employee0_.emp_id as emp_id1_1_0_, employee0_.emp_name as emp_name2_1_0_, employee0_.emp_salary as emp_sala3_1_0_, address1_.emp_id as emp_id1_0_1_, address1_.address_line1 as address_2_0_1_, address1_.city as city3_0_1_, address1_.zipcode as zipcode4_0_1_ from EMPLOYEE employee0_ left outer join ADDRESS address1_ on employee0_.emp_id=address1_.emp_id where employee0_.emp_id=?
Employee get called
Employee ID= 2
Employee Get Details:: Id= 2, Name= David, Salary= 200.0, {Address= AddressLine1= Arques Ave, City=Santa Clara, Zipcode=95051}

Employee load called
Employee ID= 1
Hibernate: select employee0_.emp_id as emp_id1_1_0_, employee0_.emp_name as emp_name2_1_0_, employee0_.emp_salary as emp_sala3_1_0_, address1_.emp_id as emp_id1_0_1_, address1_.address_line1 as address_2_0_1_, address1_.city as city3_0_1_, address1_.zipcode as zipcode4_0_1_ from EMPLOYEE employee0_ left outer join ADDRESS address1_ on employee0_.emp_id=address1_.emp_id where employee0_.emp_id=?
Employee load Details:: Id= 1, Name= Pankaj, Salary= 100.0, {Address= AddressLine1= Albany Dr, City=San Jose, Zipcode=95129}

從輸出中可以清楚地看到，get() 通過從資料庫或 hibernate 快取中獲取對象來返回對象，而 load() 只返回對象的引用，該對象實際上可能不存在，只有在您訪問對象的其他屬性時才會從資料庫或快取中加載數據。現在讓我們嘗試獲取資料庫中不存在的資料。

//get() 範例
try{
Employee emp = (Employee) session.get(Employee.class, new Long(200));
System.out.println("Employee get called");
if(emp != null){
System.out.println("Employee GET ID= "+emp.getId());
System.out.println("Employee Get Details:: "+emp+"\n");
}
}catch(Exception e){
	e.printStackTrace();
}

//load() 範例
try{
Employee emp1 = (Employee) session.load(Employee.class, new Long(100));
System.out.println("Employee load called");
System.out.println("Employee LOAD ID= "+emp1.getId());
System.out.println("Employee load Details:: "+emp1+"\n");
}catch(Exception e){
	e.printStackTrace();
}

上面的程式碼產生以下輸出。

Hibernate: select employee0_.emp_id as emp_id1_1_0_, employee0_.emp_name as emp_name2_1_0_, employee0_.emp_salary as emp_sala3_1_0_, address1_.emp_id as emp_id1_0_1_, address1_.address_line1 as address_2_0_1_, address1_.city as city3_0_1_, address1_.zipcode as zipcode4_0_1_ from EMPLOYEE employee0_ left outer join ADDRESS address1_ on employee0_.emp_id=address1_.emp_id where employee0_.emp_id=?
Employee get called
Employee load called
Employee LOAD ID= 100
Hibernate: select employee0_.emp_id as emp_id1_1_0_, employee0_.emp_name as emp_name2_1_0_, employee0_.emp_salary as emp_sala3_1_0_, address1_.emp_id as emp_id1_0_1_, address1_.address_line1 as address_2_0_1_, address1_.city as city3_0_1_, address1_.zipcode as zipcode4_0_1_ from EMPLOYEE employee0_ left outer join ADDRESS address1_ on employee0_.emp_id=address1_.emp_id where employee0_.emp_id=?
org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [com.journaldev.hibernate.model.Employee#100]
	at org.hibernate.internal.SessionFactoryImpl$1$1.handleEntityNotFound(SessionFactoryImpl.java:253)
	at org.hibernate.proxy.AbstractLazyInitializer.checkTargetState(AbstractLazyInitializer.java:262)
	at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:176)
	at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:286)
	at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:185)
	at com.journaldev.hibernate.model.Employee_$$_jvst407_1.toString(Employee_$$_jvst407_1.java)
	at java.lang.String.valueOf(String.java:2847)
	at java.lang.StringBuilder.append(StringBuilder.java:128)
	at com.journaldev.hibernate.main.HibernateExample.main(HibernateExample.java:36)

看著輸出，當我們使用 get() 來檢索不存在的數據時，它會返回 null。這是有道理的，因為它在被調用時會嘗試加載數據。使用 load()，我們能夠打印出 id，但是一旦我們嘗試訪問其他字段，它會觸發數據庫查詢並拋出 org.hibernate.ObjectNotFoundException 如果找不到具有給定標識符的記錄。這是一個特定於 Hibernate 的 運行時異常，所以我們不需要顯式地捕獲它。讓我們也來看一下一些重載的方法。上面的 get() 和 load() 方法也可以這樣寫。

Employee emp = (Employee) session.get("com.journaldev.hibernate.model.Employee", new Long(2));

Employee emp1 = (Employee) session.load("com.journaldev.hibernate.model.Employee", new Long(1));

Employee emp2 = new Employee();
session.load(emp1, new Long(1));

還有其他帶有 LockOptions 參數的方法，但我沒有使用它們。請注意，我們需要將完整的類名作為參數傳遞。根據上述解釋，我們在 get() vs load() 之間有以下區別：

1. get() 在被調用時加載數據，而 load() 返回一個代理對象，只有在實際需要時才加載數據，因此 load() 更好，因為它支持延遲加載。
2. 由於 load() 在找不到數據時會拋出異常，所以我們應該只在確定數據存在時使用它。
3. 當我們希望確保數據存在於數據庫中時，應該使用 get()。

這就是關於 Hibernate 中的 get 和 load 方法的全部內容，我希望它能消除一些疑慮，幫助您決定在不同情況下使用哪一個。