悲觀鎖和樂觀鎖是：在事務隔離機制中設置了ReadCommited的情況下，兩種可以避免不可重復讀的方式。

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設置成讀已提交是考慮到安全和處理速度，保證并發效率，但是在這個情況下仍然需要避免不可重復讀這種情況，于是hibernate提供兩種鎖來解決這個問題。

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悲觀鎖：自己事務完成之前別人不能動數據。依賴于數據庫，“for update”

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import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

@Entity
public class Account {
??? private int id;
??? private int balance; //BigDecimal
??? @Id
??? @GeneratedValue
??? public int getId() {
??????? return id;
??? }
??? public void setId(int id) {
??????? this.id = id;
??? }
??? public int getBalance() {
??????? return balance;
??? }
??? public void setBalance(int balance) {
??????? this.balance = balance;
??? }
}

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測試類：

package com.bjsxt.hibernate;

import java.math.BigDecimal;

import org.hibernate.LockMode;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class HibernateCacheTest {
??? private static SessionFactory sf;
???
??? @BeforeClass
??? public static void beforeClass() {
??????? sf = new AnnotationConfiguration().configure().buildSessionFactory();
??? }
??? @AfterClass
??? public static void afterClass() {
??????? sf.close();
??? }
???
??? @Test
??? public void testSchemaExport() {
??????? new SchemaExport(new AnnotationConfiguration().configure()).create(false, true);
??? }
???
??? @Test
??? public void testSave() {
??????? Session session = sf.openSession();
??????? session.beginTransaction();
???????
??????? Account a = new Account();
??????? a.setBalance(100);
??????? session.save(a);
???????????
??????? session.getTransaction().commit();
??????? session.close();
??? }
???
??? @Test
??? public void testOperation1() {
??????? Session session = sf.openSession();
??????? session.beginTransaction();
???????
??????? Account a = (Account)session.load(Account.class, 1);
??????? int balance = a.getBalance();
??????? //do some caculations
??????? balance = balance - 10;
??????? a.setBalance(balance);
??????? session.getTransaction().commit();
??????? session.close();
??? }
???
??? @Test
??? public void testPessimisticLock() {
??????? Session session = sf.openSession();
??????? session.beginTransaction();
???????
??????? Account a = (Account)session.load(Account.class, 1, LockMode.UPGRADE);//NOWAIT ORACL支持
??????? int balance = a.getBalance();
??????? //do some caculation
??????? balance = balance - 10;
??????? a.setBalance(balance);
??????? session.getTransaction().commit();
??????? session.close();
??? }
???
??? public static void main(String[] args) {
??????? beforeClass();
??? }
}

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樂觀鎖：依賴于表的一個字段，該字段每當該條數據被更新了，字段的內容就會變化而且不會和以前重復。將model中加上@Version即可標記該字段。

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package com.bjsxt.hibernate;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Version;

@Entity
public class Account {
??? private int id;
??? private int balance;
??? private int version;
??? @Version
??? public int getVersion() {
??????? return version;
??? }
??? public void setVersion(int version) {
??????? this.version = version;
??? }
??? @Id
??? @GeneratedValue
??? public int getId() {
??????? return id;
??? }
??? public void setId(int id) {
??????? this.id = id;
??? }
??? public int getBalance() {
??????? return balance;
??? }
??? public void setBalance(int balance) {
??????? this.balance = balance;
??? }
}

測試類：

package com.bjsxt.hibernate;

import java.math.BigDecimal;

import org.hibernate.LockMode;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class HibernateCacheTest {
??? private static SessionFactory sf;

??? @BeforeClass
??? public static void beforeClass() {
??????? sf = new AnnotationConfiguration().configure().buildSessionFactory();
??? }

??? @AfterClass
??? public static void afterClass() {
??????? sf.close();
??? }

??? @Test
??? public void testSchemaExport() {
??????? new SchemaExport(new AnnotationConfiguration().configure()).create(
??????????????? false, true);
??? }

??? @Test
??? public void testSave() {
??????? Session session = sf.openSession();
??????? session.beginTransaction();

??????? Account a = new Account();
??????? a.setBalance(100);
??????? session.save(a);

??????? session.getTransaction().commit();
??????? session.close();
??? }

??? @Test
??? public void testOptimisticLock() {
??????? Session session = sf.openSession();

??????? Session session2 = sf.openSession();

???????
???????
???????
??????? session.beginTransaction();
??????? Account a1 = (Account) session.load(Account.class, 1);
???????

??????? session2.beginTransaction();
??????? Account a2 = (Account) session2.load(Account.class, 1);
???????
??????? a1.setBalance(900);
??????? a2.setBalance(1100);

??????? session.getTransaction().commit();
??????? System.out.println(a1.getVersion());

??????? session2.getTransaction().commit();
??????? System.out.println(a2.getVersion());

??????? session.close();
??????? session2.close();
??? }

??? public static void main(String[] args) {
??????? beforeClass();
??? }
}

轉載于:https://www.cnblogs.com/flying607/p/3484241.html

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