當(dāng)前位置：首頁 > 编程资源 > 编程问答 >内容正文

编程问答

通向架构师的道路（第七天）之漫谈使用ThreadLocal改进你的层次的划分

發(fā)布時間：2024/4/14 编程问答 46 豆豆

生活随笔收集整理的這篇文章主要介紹了通向架构师的道路（第七天）之漫谈使用ThreadLocal改进你的层次的划分小編覺得挺不錯的,現(xiàn)在分享給大家,幫大家做個參考.

一、什么是ThreadLocal

早在JDK 1.2的版本中就提供Java.lang.ThreadLocal，ThreadLocal為解決多線程程序的并發(fā)問題提供了一種新的思路。使用這個工具類可以很簡潔地編寫出優(yōu)美的多線程程序。

ThreadLocal很容易讓人望文生義，想當(dāng)然地認為是一個“本地線程”。其實，ThreadLocal并不是一個Thread，而是Thread的局部變量，也許把它命名為ThreadLocalVariable更容易讓人理解一些。

當(dāng)使用ThreadLocal維護變量時，ThreadLocal為每個使用該變量的線程提供獨立的變量副本，所以每一個線程都可以獨立地改變自己的副本，而不會影響其它線程所對應(yīng)的副本。

從線程的角度看，目標變量就象是線程的本地變量，這也是類名中“Local”所要表達的意思。

線程局部變量并不是Java的新發(fā)明，很多語言（如IBM IBM XL FORTRAN）在語法層面就提供線程局部變量。在Java中沒有提供在語言級支持，而是變相地通過ThreadLocal的類提供支持。

所以，在Java中編寫線程局部變量的代碼相對來說要笨拙一些，因此造成線程局部變量沒有在Java開發(fā)者中得到很好的普及。

ThreadLocal的接口方法

ThreadLocal類接口很簡單，只有4個方法，我們先來了解一下：

2??void set(Object value)

設(shè)置當(dāng)前線程的線程局部變量的值。

2??public Object get()

該方法返回當(dāng)前線程所對應(yīng)的線程局部變量。

2??public void remove()

將當(dāng)前線程局部變量的值刪除，目的是為了減少內(nèi)存的占用，該方法是JDK 5.0新增的方法。需要指出的是，當(dāng)線程結(jié)束后，對應(yīng)該線程的局部變量將自動被垃圾回收，所以顯式調(diào)用該方法清除線程的局部變量并不是必須的操作，但它可以加快內(nèi)存回收的速度。

2??protected ObjectinitialValue()

返回該線程局部變量的初始值，該方法是一個protected的方法，顯然是為了讓子類覆蓋而設(shè)計的。這個方法是一個延遲調(diào)用方法，在線程第1次調(diào)用get()或set(Object)時才執(zhí)行，并且僅執(zhí)行1次。ThreadLocal中的缺省實現(xiàn)直接返回一個null。

值得一提的是，在JDK5.0中，ThreadLocal已經(jīng)支持泛型，該類的類名已經(jīng)變?yōu)門hreadLocal<T>。API方法也相應(yīng)進行了調(diào)整，新版本的API方法分別是void set(T value)、T get()以及T initialValue()。

一、來看一個實際案例

2.1?同一Service方法中調(diào)用多個Dao方法

可以看到，我們有一個Service方法，在該Service方法中調(diào)用多個Dao方法，所有在該Service方法中的的Dao都處于同一事務(wù)中。

該Service方法結(jié)束后，提交事務(wù)；

該Service方法中有任何錯，回滾事務(wù)；

2.2?傳統(tǒng)的做法

來看下面這段偽代碼

Service層代碼：

public void serviceMethod(){

Connection conn=null;

try{

Connection conn=getConnection();

conn.setAutoCommit(false);

Dao1 dao1=new Dao1(conn);

dao1.doSomething();

Dao2 dao2=new Dao2(conn);

dao2.doSomething();

Dao3 dao3=new Dao3(conn);

dao3.doSomething();
??????? conn.commit();

}catch(Exception e){

??? try{

??? conn.rollback();

}catch(Exception ex){}

}finally{

try{

conn.setAutoCommit(true);

}catch(Exception e){}

??? try{

??? if(conn!=null){

??? conn.close();

??? conn=null;

}

}catch(Exception e){}

}

每個Dao層的代碼：

Class Dao1{

private Connection conn=null;

public Dao1(Connection conn){

??? this.conn=conn;

}

public void doSomething(){

??? PreparedStatement pstmt=null;

??? try{

??????? pstmt=conn.preparedStatement(sql);

??????? pstmt.execute…

??????? …

}catch(Exception e){

??? log.error(e,”Exeception occurred in Dao1.doSomething():”+e.getMessage,e);

}finally{

??? try{

??????? if(pstmt!=null){

??????????? pstmt.close();

??????????? pstmt=null;

}

??? }catch(Exception e){}

}

如果我一個Service方法有調(diào)用一堆dao方法，先不說這樣寫首先破壞了OOP的封裝性原則，如果有一個dao多關(guān)了一個conn，那就會導(dǎo)致其它的dao得到的conn為null，這種事在這樣的寫法下由其當(dāng)你還有業(yè)務(wù)邏輯混合在一起時很容易發(fā)生。

筆者曾經(jīng)遇見過2個項目，出現(xiàn)out of memory或者是connection pool has been leakage，經(jīng)查代碼就是在每個dao中多關(guān)或者在service層中漏關(guān)，或者是每個dao有自己的conntionconn=getConnection()，然后還跑到Service層里去關(guān)這個connection（那關(guān)什么，關(guān)個P關(guān)！）。

當(dāng)然，如果你說你在寫法上絕對promise絕對注意這樣的問題不會發(fā)生，但是我們來看看下面的這種做法，是否會比上面這個寫法更好呢？

2.3 Spring中的做法

先來看spring中的寫法。

大家應(yīng)該都很熟悉Spring中的寫法了，來看一下它是怎么解決的。

Service層

public void serviceMethod(){

try{

??? //aop 自動加入connection，并且將conn.setAutoCommit(false);

dao1.doSomething();

dao2.doSomething();

dao3.doSomething();

}catch(Exception e){

??? //aop 自動加入rollback

}finally{

??? //aop自動加入conn.setAutoCommit(true)

??? //aop 自動加入conn.close();

}

這邊我們不講AOP，因為用類反射結(jié)合xml很容易將aop 自動。。。這些東西加入我們的代碼中去是不是？我們只管寫dao方法，service方法，不需要關(guān)心在哪邊commit哪邊rollback何時connection，spring的聲明式事務(wù)會幫我們負責(zé)，這種風(fēng)格我們稱為“優(yōu)雅”，各層間耦合度極大程度上的降低，封裝性好。

因此，我們可以總結(jié)出下面這些好處：

2? Service層的方法只管開啟事務(wù)（如果講究點的還會設(shè)一個Transaction）；

2? 在該Service層中的所有dao使用該service方法中開啟的事務(wù)（即connection）；

2? Dao中每次只管getCurrentConnection（獲取當(dāng)前的connection），與進行數(shù)據(jù)處理

2? Dao層中如果發(fā)生錯誤就拋回Service層

2? Service層中接到exception，在catch{}中rollback，在try{}未尾commit，在finally塊中關(guān)閉整個connection。

這。。。就是我們所說的ThreadLocal。

舉個更實際的例子再次來說明ThreadLocal：

我們有3個用戶訪問同一個service方法，該service方法內(nèi)有3個dao方法為一個完整事務(wù)，那么整個web容器內(nèi)只因該有3個connection，并且每個connection之間的狀態(tài)，彼此“隔離”。

我們下面一起來看我們?nèi)绾斡么a實現(xiàn)類似于Spring的這種做法。

首先，根據(jù)我們的ThreadLocal的概念，我們先聲明一個ConnectionManager的類。

2.4?利用ThreadLocal制作ConnectionManager

public class ConnectionManager {

???????? private static ThreadLocal tl = new ThreadLocal();

???????? private static Connection conn = null;

???????? public static void BeginTrans(boolean beginTrans) throws Exception {

?????????????????? if (tl.get() == null || ((Connection) tl.get()).isClosed()) {

??????????????????????????? conn = SingletonDBConnection.getInstance().getConnection();

??????????????????????????? conn = new ConnectionSpy(conn);

??????????????????????????? if (beginTrans) {

???????????????????????????????????? conn.setAutoCommit(false);

??????????????????????????? }

??????????????????????????? tl.set(conn);

?????????????????? }

???????? }

???????? public static Connection getConnection() throws Exception {

?????????????????? return (Connection) tl.get();

???????? }

???????? public static void close() throws SQLException {

?????????????????? try {

??????????????????????????? ((Connection) tl.get()).setAutoCommit(true);

?????????????????? } catch (Exception e) {

?????????????????? }

?????????????????? ((Connection) tl.get()).close();

?????????????????? tl.set(null);

???????? }

???????? public static void commit() throws SQLException {

?????????????????? try {

??????????????????????????? ((Connection) tl.get()).commit();

?????????????????? } catch (Exception e) {

?????????????????? }

?????????????????? try {

??????????????????????????? ((Connection) tl.get()).setAutoCommit(true);

?????????????????? } catch (Exception e) {

?????????????????? }

???????? }

???????? public static void rollback() throws SQLException {

?????????????????? try {

??????????????????????????? ((Connection) tl.get()).rollback();

?????????????????? } catch (Exception e) {

?????????????????? }

?????????????????? try {

??????????????????????????? ((Connection) tl.get()).setAutoCommit(true);

?????????????????? } catch (Exception e) {

?????????????????? }

???????? }

}

2.5?利用ThreadLocal改造Service與Dao層

Service層（注意紅色標粗-好粗yeah，的地方）

package sky.org.service.impl;

public class StudentServiceImpl implements StudentService {

???????? public void addStudent(Student std) throws Exception {

???????????????????StudentDAO studentDAO = new StudentDAOImpl();

?????????????????? ClassRoomDAO classRoomDAO = new ClassRoomDAOImpl();

?????????????????? try {

??????????????????????????? ConnectionManager.BeginTrans(true);

????????????????????????????studentDAO.addStudent(std);

????????????????????????????classRoomDAO

??????????????????????????????????????????????.addStudentClassRoom(std.getClassRoomId(), std.getsNo());

????????????????????????????ConnectionManager.commit();

?????????????????? } catch (Exception e) {

????????????????????????????try {

???????????????????????????????????? ConnectionManager.rollback();

??????????????????????????? } catch (Exception de) {

??????????????????????????? }

??????????????????????????? throw new Exception(e);

?????????????????? }finally {

??????????????????????????? try {

???????????????????????????????????? ConnectionManager.close();

??????????????????????????? } catch (Exception e) {

??????????????????????????? }

?????????????????? }

???????? }

}

Look，如果我把上述標粗（沒有加紅色）的地方，全部用AOP的方式從這塊代碼的外部“切”進去。。。是不是一個Spring里的Service方法就誕生了？

下面來看一個完整的例子

2.6?使用ThreadLocal分離Service、DAO層

先來看表結(jié)構(gòu)：

T_Student表

T_ClassRoom表

T_Student_ClassRoom表

需求：

很簡單，T_ClassRoom表里已經(jīng)有值了，在插入T_Student表的數(shù)據(jù)時同時要給這個學(xué)生分配一個班級并且插入T_Student_ClassRoom表，這就是一個事務(wù)，這兩步中有任何一步出錯，事務(wù)必須回滾。

看來工程的結(jié)構(gòu)吧：

下面開始放出所有源代碼：

2.6.1 ConnectionManager類

package sky.org.util.db;

import java.sql.*;

public class ConnectionManager {

???????? private static ThreadLocal tl = new ThreadLocal();

???????? private static Connection conn = null;

???????? public static void BeginTrans(boolean beginTrans) throws Exception {

?????????????????? if (tl.get() == null || ((Connection) tl.get()).isClosed()) {

??????????????????????????? conn = DBConnection.getInstance().getConnection();

??????????????????????????? conn = new ConnectionSpy(conn);

??????????????????????????? if (beginTrans) {

???????????????????????????????????? conn.setAutoCommit(false);

??????????????????????????? }

??????????????????????????? tl.set(conn);

?????????????????? }

???????? }

???????? public static Connection getConnection() throws Exception {

????????????????? return (Connection) tl.get();

???????? }

???????? public static void close() throws SQLException {

?????????????????? try {

??????????????????????????? ((Connection) tl.get()).setAutoCommit(true);

?????????????????? } catch (Exception e) {

?????????????????? }

?????????????????? ((Connection) tl.get()).close();

?????????????????? tl.set(null);

???????? }

???????? public static void commit() throws SQLException {

?????????????????? try {

??????????????????????????? ((Connection) tl.get()).commit();

?????????????????? } catch (Exception e) {

?????????????????? }

?????????????????? try {

??????????????????????????? ((Connection) tl.get()).setAutoCommit(true);

?????????????????? } catch (Exception e) {

?????????????????? }

???????? }

???????? public static void rollback() throws SQLException {

?????????????????? try {

??????????????????????????? ((Connection) tl.get()).rollback();

?????????????????? } catch (Exception e) {

?????????????????? }

?????????????????? try {

??????????????????????????? ((Connection) tl.get()).setAutoCommit(true);

?????????????????? } catch (Exception e) {

?????????????????? }

???????? }

}

2.6.2 DBConnection類

package sky.org.util.db;

public class DBConnection {

???????? private static DBConnection instance = null;

???????? private static String driverClassName = null;

???????? private static String connectionUrl = null;

???????? private static String userName = null;

???????? private static String password = null;

???????? private static Connection conn = null;

???????? private static Properties jdbcProp = null;

???????? private DBConnection() {

???????? }

???????? private static Properties getConfigFromPropertiesFile() throws Exception {

?????????????????? Properties prop = null;

?????????????????? prop = JdbcProperties.getPropObjFromFile();

?????????????????? return prop;

???????? }

???????? private static void initJdbcParameters(Properties prop) {

?????????????????? driverClassName = prop.getProperty(Constants.DRIVER_CLASS_NAME);

?????????????????? connectionUrl = prop.getProperty(Constants.CONNECTION_URL);

?????????????????? userName = prop.getProperty(Constants.DB_USER_NAME);

?????????????????? password = prop.getProperty(Constants.DB_USER_PASSWORD);

???????? }

???????? private static void createConnection() throws Exception {

?????????????????? Class.forName(driverClassName);

?????????????????? conn = DriverManager.getConnection(connectionUrl, userName, password);

???????? }

???????? public static Connection getConnection() throws Exception {

?????????????????? return conn;

???????? }

???????? public synchronized static DBConnection getInstance()throws Exception{

?????????????????? if (instance == null) {

??????????????????????????? jdbcProp = getConfigFromPropertiesFile();

??????????????????????????? instance = new DBConnection();

?????????????????? }

?????????????????? initJdbcParameters(jdbcProp);

?????????????????? createConnection();

?????????????????? return instance;

???????? }

}

2.6.3 JdbcProperties類

package sky.org.util.db;

import java.io.File;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

import java.io.IOException;

import java.io.InputStream;

import java.NET.URL;

import java.util.*;

public class JdbcProperties {

???????? private static Log logger = LogFactory.getLog(JdbcProperties.class);

???????? public static Properties getPropObjFromFile() {

?????????????????? Properties objProp = new Properties();

?????????????????? ClassLoader classLoader = Thread.currentThread()

???????????????????????????????????? .getContextClassLoader();

?????????????????? URL url = classLoader.getResource(Constants.JDBC_PROPERTIES_FILE);

?????????????????? if (url == null) {

??????????????????????????? classLoader = ClassLoader.getSystemClassLoader();

??????????????????????????? url = classLoader.getResource(Constants.JDBC_PROPERTIES_FILE);

?????????????????? }

?????????????????? File file = new File(url.getFile());

?????????????????? InputStream inStream = null;

?????????????????? try {

??????????????????????????? inStream = new FileInputStream(file);

??????????????????????????? objProp.load(inStream);

?????????????????? } catch (FileNotFoundException e) {

??????????????????????????? objProp = null;

??????????????????????????? e.printStackTrace();

?????????????????? } catch (IOException e) {

??????????????????????????? e.printStackTrace();

?????????????????? } finally {

??????????????????????????? try {

???????????????????????????????????? if (inStream != null) {

?????????????????????????????????????????????? inStream.close();

?????????????????????????????????????????????? inStream = null;

???????????????????????????????????? }

??????????????????????????? } catch (Exception e) {

??????????????????????????? }

?????????????????? }

?????????????????? return objProp;

???????? }

}

2.6.4 Resource目錄下的jdbc.properties

jdbc.driverClassName=com.MySQL.jdbc.Driver

jdbc.databaseURL=jdbc:mysql://localhost:3306/mydb?useUnicode=true&characterEncoding=utf8

jdbc.username=mysql

jdbc.password=password_1

2.6.5 StudentService接口

package sky.org.service;

import java.util.List;

import java.util.Vector;

import sky.org.bean.*;

public interface StudentService {

???????? public void addStudent(Student std) throws Exception;

}

2.6.6 StudentServiceImpl類

package sky.org.service.impl;

import java.util.ArrayList;

import java.util.List;

import java.util.Vector;

import sky.org.util.db.ConnectionManager;

import sky.org.util.*;

import sky.org.bean.*;

import sky.org.dao.*;

import sky.org.dao.impl.*;

import sky.org.service.*;

public class StudentServiceImpl implements StudentService {

???????? public void addStudent(Student std) throws Exception {

?????????????????? StudentDAO studentDAO = new StudentDAOImpl();

?????????????????? ClassRoomDAO classRoomDAO = new ClassRoomDAOImpl();

?????????????????? try {

??????????????????????????? ConnectionManager.BeginTrans(true);

??????????????????????????? studentDAO.addStudent(std);

??????????????????????????? classRoomDAO

?????????????????????????????????????????????? .addStudentClassRoom(std.getClassRoomId(), std.getsNo());

??????????????????????????? ConnectionManager.commit();

?????????????????? } catch (Exception e) {

??????????????????????????? try {

???????????????????????????????????? ConnectionManager.rollback();

??????????????????????????? } catch (Exception de) {

??????????????????????????? }

??????????????????????????? throw new Exception(e);

?????????????????? } finally {

??????????????????????????? try {

???????????????????????????????????? ConnectionManager.close();

??????????????????????????? } catch (Exception e) {

??????????????????????????? }

?????????????????? }

???????? }

}

2.6.7 ClassRoomDAO接口

package sky.org.dao;

import java.util.HashMap;

import java.util.List;

public interface ClassRoomDAO {

???????? public void addStudentClassRoom(String roomId, String sNo) throws Exception;

}

2.6.8 ClassRoomDAOImpl類

package sky.org.dao.impl;

import java.sql.*;

import java.util.*;

import sky.org.dao.ClassRoomDAO;

import sky.org.util.db.ConnectionManager;

public class ClassRoomDAOImpl implements ClassRoomDAO {

???????? public void addStudentClassRoom(String roomId, String sNo) throws Exception {

?????????????????? Connection conn = null;

?????????????????? PreparedStatement pstmt = null;

?????????????????? try {

??????????????????????????? conn = ConnectionManager.getConnection();

??????????????????????????? pstmt = conn

?????????????????????????????????????????????? .prepareStatement(ClassRoomDAOSql.ADD_STUDENT_CLASSROOM);

??????????????????????????? pstmt.setString(1, roomId);

??????????????????????????? pstmt.setString(2, sNo);

??????????????????????????? pstmt.executeUpdate();

?????????????????? } catch (Exception e) {

??????????????????????????? throw new Exception("addStudentClassRoom:" + e.getMessage(), e);

?????????????????? } finally {

??????????????????????????? try {

???????????????????????????????????? if (pstmt != null) {

?????????????????????????????????????????????? pstmt.close();

?????????????????????????????????????????????? pstmt = null;

???????????????????????????????????? }

??????????????????????????? } catch (Exception e) {

??????????????????????????? }

?????????????????? }

???????? }

}

2.6.9 StudentDAO接口

package sky.org.dao;

import java.util.*;

import sky.org.bean.Student;

public interface StudentDAO {

???????? public void addStudent(Student std) throws Exception;

}

2.6.10 StudentDAOImpl類

package sky.org.dao.impl;

import java.sql.*;

import javax.sql.*;

import org.apache.commons.logging.Log;

import org.apache.commons.logging.LogFactory;

import sky.org.bean.Student;

import sky.org.dao.StudentDAO;

import sky.org.util.db.ConnectionManager;

import java.util.List;

import java.util.ArrayList;

import java.util.Vector;

import java.text.*;

import sky.org.util.StringUtil;

public class StudentDAOImpl implements StudentDAO {

???????? private Log logger = LogFactory.getLog(this.getClass());

???????? public void addStudent(Student std) throws Exception {

?????????????????? Connection conn = null;

?????????????????? PreparedStatement pstmt = null;

?????????????????? try {

??????????????????????????? conn = ConnectionManager.getConnection();

??????????????????????????? pstmt = conn.prepareStatement(StudentDAOSql.ADD_STUDENT);

??????????????????????????? pstmt.setString(1, std.getsNo());

??????????????????????????? pstmt.setString(2, std.getsName());

??????????????????????????? pstmt.setString(3, std.getsAge());

??????????????????????????? pstmt.setString(4, std.getGender());

??????????????????????????? pstmt.setDate(5, StringUtil.convertStrToDate(std.getSbirth()));

??????????????????????????? pstmt.executeUpdate();

?????????????????? } catch (Exception e) {

??????????????????????????? throw new Exception("addStudent:" + e.getMessage(), e);

?????????????????? } finally {

??????????????????????????? try {

???????????????????????????????????? if (pstmt != null) {

?????????????????????????????????????????????? pstmt.close();

?????????????????????????????????????????????? pstmt = null;

???????????????????????????????????? }

??????????????????????????? } catch (Exception e) {

??????????????????????????? }

?????????????????? }

???????? }

???????? public void delStudent(String sNo) throws Exception {

?????????????????? Connection conn = null;

?????????????????? PreparedStatement pstmt = null;

?????????????????? try {

??????????????????????????? conn = ConnectionManager.getConnection();

??????????????????????????? pstmt = conn.prepareStatement(StudentDAOSql.DEL_STUDENT);

??????????????????????????? pstmt.setString(1, sNo);

??????????????????????????? pstmt.executeUpdate();

?????????????????? } catch (Exception e) {

??????????????????????????? throw new Exception("delStudent:" + e.getMessage(), e);

?????????????????? } finally {

??????????????????????????? try {

???????????????????????????????????? if (pstmt != null) {

?????????????????????????????????????????????? pstmt.close();

?????????????????????????????????????????????? pstmt = null;

???????????????????????????????????? }

??????????????????????????? } catch (Exception e) {

??????????????????????????? }

?????????????????? }

???????? }

}

2.6.11 StudentDAOSql類

package sky.org.dao.impl;

public class StudentDAOSql {

public final static String ADD_STUDENT = "insert into t_student(sno, sname, sage, gender,

sbirth)values(?,?,?,?,?)";

}

2.6.12 ClassRoomDAOSql類

package sky.org.dao.impl;

public class ClassRoomDAOSql {

???????? public static String ADD_STUDENT_CLASSROOM = "insert into

t_student_classroom(room_id,sno)values(?,?)";

}

2.6.13 ClassRoom?類

package sky.org.bean;

import java.io.*;

public class ClassRoom implements Serializable {

???????? private String roomId = "";

???????? private String roomName = "";

???????? public String getRoomId() {

?????????????????? return roomId;

???????? }

???????? public void setRoomId(String roomId) {

?????????????????? this.roomId = roomId;

???????? }

???????? public String getRoomName() {

?????????????????? return roomName;

???????? }

???????? public void setRoomName(String roomName) {

?????????????????? this.roomName = roomName;

???????? }

}

2.6.14 Student類

package sky.org.bean;

import java.io.*;

public class Student implements Serializable {

???????? public String getsNo() {

?????????????????? return sNo;

???????? }

???????? public void setsNo(String sNo) {

?????????????????? this.sNo = sNo;

???????? }

???????? public String getsName() {

?????????????????? return sName;

???????? }

???????? public void setsName(String sName) {

?????????????????? this.sName = sName;

???????? }

???????? public String getsAge() {

?????????????????? return sAge;

???????? }

???????? public void setsAge(String sAge) {

?????????????????? this.sAge = sAge;

???????? }

???????? public String getGender() {

?????????????????? return gender;

???????? }

???????? public void setGender(String gender) {

?????????????????? this.gender = gender;

???????? }

???????? private String sNo = "";

???????? private String sName = "";

???????? private String sAge = "";

???????? private String gender = "";

???????? private String sbirth = "";

???????? private String classRoomId = "";

???????? private String classRoomName = "";

???????? public String getClassRoomId() {

?????????????????? return classRoomId;

???????? }

???????? public void setClassRoomId(String classRoomId) {

?????????????????? this.classRoomId = classRoomId;

???????? }

???????? public String getClassRoomName() {

?????????????????? return classRoomName;

???????? }

???????? public void setClassRoomName(String classRoomName) {

?????????????????? this.classRoomName = classRoomName;

???????? }

???????? public String getSbirth() {

?????????????????? return sbirth;

???????? }

???????? public void setSbirth(String sbirth) {

?????????????????? this.sbirth = sbirth;

???????? }

}

2.6.15 StudentCRUD類（運行主類）

package sky.org.test;

import sky.org.bean.Student;

import sky.org.service.StudentService;

import sky.org.service.impl.StudentServiceImpl;

public class StudentCRUD {

???????? public void addStudent() throws Exception {

?????????????????? StudentService stdService = new StudentServiceImpl();

?????????????????? Student std = new Student();

?????????????????? std.setsNo("101");

?????????????????? std.setsName("abc");

?????????????????? std.setSbirth("1977/01/01");

?????????????????? std.setsAge("35");

?????????????????? std.setGender("m");

?????????????????? std.setClassRoomId("1");

?????????????????? std.setClassRoomName("class1");

?????????????????? stdService.addStudent(std);

???????? }

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

?????????????????? StudentCRUD testStudentCRUD = new StudentCRUD();

?????????????????? try {

??????????????????????????? testStudentCRUD.addStudent();

?????????????????? } catch (Exception e) {

??????????????????????????? e.printStackTrace();

??????????????????????????? System.exit(-1);

?????????????????? }

???????? }

}

三、Hibernate里的ThreadLocal

hibernate在事務(wù)操作中也支持ThreadLocal的作法，我們這邊指的是不用Spring去做代理，而直接用Hibernate。即：

Service Method{

hbDAO1.doSomething();

hbDAO2.doSomething();

hbDAO3.doSomething();

。。。

}

Hibernate版本3后增加了新特性，即getCurrentSession()。

3.1 getCurrentSession與openSession的區(qū)別

3.1.1 openSession

我們傳統(tǒng)的做法是openSession即：

public void testUser() throws Exception {

?????????????????? Transaction tran = null;

?????????????????? SessionFactory factory = null;

?????????????????? UserDAO userDAO = new UserDAOImpl();

?????????????????? try {

??????????????????????????? factory = HibernateUtil.getInstance().getSessionFactory();

??????????????????????????? Session session = factory.openSession();

??????????????????????????? tran = session.beginTransaction();

??????????????????????????? TUser testUser = new TUser();

??????????????????????? testUser.setId(new Integer(i));

??????????????????????????? testUser.setName("abc");

??????????????????????????? userDAO.addUser(testUser);

??????????????????????????? tran.commit();

?????????????????? } catch (Exception e) {

??????????????????????????? tran.rollback();

??????????????????????????? throw new Exception(e);

?????????????????? } finally {

??????????????????????????? try{

??? if(session!=null){

??? session.close();

??? session=null();

}

}catch(Excepton e){}

?????????????????? }

???????? }

這樣做，能夠保證我們每次在finally塊中正確關(guān)閉session，但是，如果我們也遇上了這樣的case即：

Service Method{

hbDAO1.doSomething();

hbDAO2.doSomething();

hbDAO3.doSomething();

。。。

}

這時，我們?nèi)绻玫氖莖penSession，應(yīng)該怎么辦？

解決方案一：

自己用ThreadLocal模式寫一個SessionManagement類，來維護這個session。

解決方案二：

把在Service方法中打開的session，傳到每個dao方法中，使每個dao方法使用同一個session，最后在Service方法中去關(guān)閉它（很爛的做法）。

下面我們來看看Hibernate自身提供的getCurrentSession()的做法吧

3.1.2 getCurrentSession

要使用這個getCurrentSession，你的hibernate的設(shè)置必須如下（紅色加粗部分顯示-就喜歡粗J）：

<?xml version='1.0' encoding='UTF-8'?>

<!DOCTYPE hibernate-configuration PUBLIC

????????? "-//Hibernate/Hibernate Configuration DTD 3.0//EN"

????????? "http://hibernate.sourceforge.Net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

???????? <session-factory>

?????????????????? <property name="connection.url">

??????????????????????????? jdbc:Oracle:thin:@localhost:1521:myorcl

?????????????????? </property>

?????????????????? <property name="dialect">

??????????????????????????? org.hibernate.dialect.Oracle9Dialect

?????????????????? </property>

?????????????????? <property name="connection.username">abc</property>

?????????????????? <property name="connection.password">abc</property>

?????????????????? <property name="connection.driver_class">

??????????????????????????? oracle.jdbc.OracleDriver

?????????????????? </property>

?????????????????? <property name="show_sql">true</property>

?????????????????? <property name="hibernate.hbm2ddl.auto">update</property>

???????????????????<property name="hibernate.current_session_context_class">thread</property>

?????????????????? <mapping resource="com/cts/testhb/model/TUser.hbm.xml" />

???????? </session-factory>

</hibernate-configuration>

然后上述代碼將變成如下的樣子：

public void testUser() throws Exception {

?????????????????? Transaction tran = null;

?????????????????? SessionFactory factory = null;

?????????????????? UserDAO userDAO = new UserDAOImpl();

?????????????????? try {

??????????????????????????? factory = HibernateUtil.getInstance().getSessionFactory();

????????????????????????????Session session = factory.getCurrentSession();

??????????????????????????? tran = session.beginTransaction();

??????????????????????????? for (int i = 0; i < 100; i++) {

???????????????????????????????????? TUser testUser = new TUser();

???????????????????????????????????? testUser.setId(new Integer(i));

???????????????????????????????????? testUser.setName("abc");

???????????????????????????????????? userDAO.addUser(testUser);

??????????????????????????? }

??????????????????????????? tran.commit();

?????????????????? } catch (Exception e) {

??????????????????????????? tran.rollback();

??????????????????????????? throw new Exception(e);

?????????????????? } finally {

????????????????????????????ThreadLocalSessionContext.unbind(factory);

?????????????????? }

???????? }

而你的每個DAO方法中的代碼是這樣實現(xiàn)的：

public void addUser(TUser user) throws Exception {

???????? SessionFactory factory = HibernateUtil.getInstance()

??????????????????????????? .getSessionFactory();

???????? Session session = factory.getCurrentSession();

???????? session.save(user);

}

是不是很方便的哈。

3.1.3 openSession與getCurrentSession的區(qū)別

嚴重注意下面3點：

2? openSession一旦被調(diào)用，必須且一定要在finally塊中close，要不然你就等著out of memory吧；

2? 如果你使用的是getCurrentSession，那么你不能在finally塊中調(diào)用”session.close()”，不行你可以在finally塊中用try-catch把session.close();包起來，然后在catch{}塊中拋出這個exception，這個exception將會是：sessionhas been already closed。

因為：

l?? 如果你用的是getCurrentSession，那么它在session.commit()或者是session.rollback()時就已經(jīng)調(diào)用了一次session.close()了，因此你只要正確放置session.commit()與rollback()即可。

l?? 你必須在finally塊中調(diào)用”ThreadLocalSessionContext.unbind(factory);”，以使得當(dāng)前的事務(wù)結(jié)束時把session（即dbconnection）還回db connection pool中

2? 如果你使用的是getCurrentSession，那么就算你是一個簡單的select語句，也必須包含在：

tran = session.beginTransaction();

//your select hibernate query

tran.commit();

這樣的事務(wù)塊中，要不然它將會拋出這樣的一個錯誤：

NoHibernate Session bound to thread, and configuration does not allow creation ofnon-transactional

看下面的例子：

try {

??????????????????? factory = HibernateUtil.getInstance().getSessionFactory();

??????????????????? Session session = factory.getCurrentSession();

????????????????????tran = session.beginTransaction();

??????????????????? TUser testUser = userDAO.getUserByID("1");

??????????????????? log.info("user id===="+testUser.getId()+"? user name===="+testUser.getName());

????????????????????tran.commit();

?????????? } catch (Exception e) {

????????????????????tran.rollback();

??????????????????? throw new Exception(e);

?????????? } finally {

??????????????????? ThreadLocalSessionContext.unbind(factory);

?????????????????? }

可以看到我們的查詢是被tran=session.beginTransaction一直到tran.commit()或者是tran.rollback()結(jié)束的，如果，你把你的hibernate查詢移到了tran=session.beginTransaction的上面。。。就會拋上述這個錯誤。

3.1.4 getCurrentSession帶來的問題

getCurrentSession非常好，不需要我們自己寫ThreadLocal只需要在hibernate.cfg的配置文件中聲音一下就可以獲得ThreadLocal的好處，便于我們劃分我們的程序的層次與封裝，帶也帶來了一定的性能問題。

特別是“如果你使用的是getCurrentSession，那么就算你是一個簡單的select語句，也必須包含在事務(wù)塊中”。這給我們帶來了很大的問題。

因此，本人建議，在碰到如果：

1.?????? 一個service方法中只有單個dao操作且此操作是一個select類的操作，請使用openSession，并且即時在finally塊中關(guān)閉它；

2.?????? 如果一個service方法中涉及到多個dao操作，請一定使用getCurrentSession；

3.?????? 如果一個service方法中混合著select操作，delete, update, insert操作。請按照下述原則：

1)????? 將屬于select的操作，單獨做成一個dao方法，該dao使用openSession并且在finally塊中及時關(guān)閉session,該dao只需要返回一個java的object如：List<Student>即可，如果出錯將exception拋回給調(diào)用它的service方法。

2)????? 對于其它的delete, insert, update的dao操作，請使用getCurrentSession。

4.?????? 忌諱，把select類的操作放在“事務(wù)”中；

總結(jié)

以上是生活随笔為你收集整理的通向架构师的道路（第七天）之漫谈使用ThreadLocal改进你的层次的划分的全部內(nèi)容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網(wǎng)站內(nèi)容還不錯，歡迎將生活随笔推薦給好友。

上一篇：通向架构师的道路（第六天）之漫谈基于数据
下一篇：通向架构师的道路（第十天）之Axis2