事务管理（三）——JDBC事务管理的简单实现

概述：
为了学习事务的管理，实现了如下的一个简单版本的事务管理器，进行实验。该管理器简化了事务管理的各种异常类型的考虑。

对于Transaction 管理
为了实现数据一致性，对于数据库的JDBC编程通常需要在代码中显示的调用Connection方法的事务相关API来完成工作。
常见的代码如下：

public void doService(){
    Connection connection = getConnection();
    connection.setTransactionIsolation(
    Connection.TRANSACTION_READ_COMMITTED);
    try{
        doBusinessLogic(connection);
        connection.commit();
    }catch(Exception e){
        connection.rollback();
    }finally{
        connection.close();
    }
}

在以上的代码中与业务逻辑相关的代码只有一行， 为了简化事务的操作，实现一个简单的事务管理控制。主要提供以下方面的统一控制。
1．使用回调机制简化事务处理的代码
2．增加对事务隔离级别的支持
3．增加事务Timeout的管理，最简单的实现。 尽早发现长事务的操作。
4．对于savepoint的支持
5．实现一种事务传播控制。 如果当前存在事务，就加入该事务中执行，如果没有新建事务，放到事务中实现。

实现了该事务控制管理器后，将Service的代码得到如下的简化：

private Service service;
private Dao dao;
private TransactionTemplate transactionTemplate;
public void service(final Object obj) {
    this.trTemplate.executeTransaction(new TransactionCallback(){
	  public Object doTransactionEvent() {
		service.doService();
		transactionTemplate.addSavePoint("savepoint1", exception1);
		dao.saveMethod1(obj);
		transactionTemplate.addSavePoint("savepoint2", exception2);
		dao.saveMethod2(obj);
	  }
    });		
}

1. 回调的实现
定义模板类TransactionTemplate
其主要的方法就是提供模板方法对事务处理的代码进行控制：

public class TransactionTemplate {
	
	private TransactionManager trManager;
	
	//the timeout monitor is used to find out all long time transaction in advance.
	private boolean useTimeoutMonitor = Boolean.TRUE;
	private Long maxTime = Long.valueOf(2);
	
	public void addSavePoint(String savePointName, Exception rollbackException){
		this.trManager.addSavePoint(savePointName, rollbackException);
	}
	
	public void executeTransaction(
  TransactionCallback transactionCallback, int isolationLevel) {
		long startTime = System.currentTimeMillis();
		Transaction transaction = null;
		try{
			transaction = trManager.beginTransaction();
			checkTimeout(startTime);
			transaction.setTransactionIsolationLevel(isolationLevel);
			transactionCallback.doTransactionEvent();
			checkTimeout(startTime);
			trManager.commitTransaction(transaction);
		}catch(Exception e){
			trManager.rollbackTransaction(transaction, e);
			
		}finally{
			trManager.closeTransaction(transaction);
		}
		
	}

	/**
	 * If the transaction is timeout throw a transaction time out exception.
	 */
	private void checkTimeout(long startTime) {
		if( this.useTimeoutMonitor ){
			if(isTimeout(startTime)){
				throw new TransactionTimeoutException();
			}
		}
	}
	
	private boolean isTimeout(long startTime) {
		return System.currentTimeMillis() - startTime > this.maxTime ;
	}

	public void executeTransaction(TransactionCallback transactionCallback){
		this.executeTransaction(transactionCallback, Transaction.DEFAULT_ISOLATION_LEVEL);
	
	}

	public Long getMaxTime() {
		return maxTime;
	}

	public void setMaxTime(Long maxTime) {
		this.maxTime = maxTime;
	}

}

2． 回调接口的定义非常的简单：

public interface TransactionCallback {
	public Object doTransactionEvent();
}

3． 重点Transaction类的实现，管理了Connection对象，事务隔离级别的定义，事务的savepoint控制点，以及对于事务状态的控制。

public class Transaction {
	public static final int READ_UNCOMMITTED = Connection.TRANSACTION_READ_UNCOMMITTED;	
	public static final int READ_COMMITTED = Connection.TRANSACTION_READ_COMMITTED; 
	public static final int REPEATALB_READ = Connection.TRANSACTION_REPEATABLE_READ;
	public static final int SERIALIZE = Connection.TRANSACTION_SERIALIZABLE;
	public static final int DEFAULT_ISOLATION_LEVEL = READ_COMMITTED;
	
	private Map<Exception,Savepoint> savePoints = new HashMap<Exception, Savepoint>();
	
	//This count is used to implement the transaction propagation  
	private int newTrCount = 1;

	Connection connection;
	
	
	public Transaction(Connection connection){
		this.connection = connection;
	}
	
	public void addNewService(){
		this.newTrCount++;
	}
	
	public void completeService(){
		this.newTrCount--;
	}
	
	public boolean isNewTransaction(){
		return this.newTrCount == 1;
	}

	/**
	 * @return the connection
	 */
	public Connection getConnection() {
		return connection;
	}

	/**
	 * @param connection the connection to set
	 */
	public void setConnection(Connection connection) {
		this.connection = connection;
	}

	/**
	 * @param transactionIsolationLevel the transactionIsolationLevel to set
	 * @throws SQLException 
	 */
	public void setTransactionIsolationLevel(int transactionIsolationLevel) throws SQLException {
		this.connection.setAutoCommit(false);
		this.connection.setTransactionIsolation(transactionIsolationLevel);
	}
	
	public void addSavepointAndRollbackException(String name, Exception e) 
		throws SQLException{
		Savepoint savepoint = this.addSavePoint(name);
		this.savePoints.put(e, savepoint);
	}
	
	public boolean isSupportSavepoint() throws SQLException{
		return this.getConnection().getMetaData().supportsSavepoints();
	}
	
	private Savepoint addSavePoint(String savepointName) throws SQLException{
		return this.connection.setSavepoint(savepointName);
	}

	public boolean containsSavepoint() {
		return !this.savePoints.isEmpty();
	}

	public Savepoint getSavePointByException(Exception e) {
		return this.savePoints.get(e);
	}

}

1） 对于savePoints属性记录了所有的事务的保存点，当service运行抛出Exception时，将进入rollback逻辑，这个时候，rollback方法会查找相应的Exception的保存点，将事务回滚到该保存点。
具体的保存点检查代码如下：
public void addSavepointAndRollbackException(String name, Exception e)

public boolean isSupportSavepoint() throws SQLException{}

private Savepoint addSavePoint(String savepointName) throws SQLException{}

public boolean containsSavepoint() {}

public Savepoint getSavePointByException(Exception e) {}

2） 属性newTrCount主要是用于实现事务的传播控制。当且仅当该值为1时候表示该事务是新创建的一个事务。当一个service运行的时候，如果发现当前的service线程已经有事务对象，那么这个时候就不需要再重新创建事务，只需要将原有的事务对象的计数器加1，那么相应的当该Serice运行完成才将事务减一，从而保证只有最外层的service看到的事务对象是新创建的事务对象，会执行真正的事务操作，而其他嵌套的Service对象，只是对该count进行操作。
提供了如下的三个方法方便控制：
public void addNewService(){
    this.newTrCount++;
}

public void completeService(){
    this.newTrCount--;
}

public boolean isNewTransaction(){
    return this.newTrCount == 1;
}

4． 核心对象TransactionManager 该对象管理了Transaction对象，并且控制着事务的操作。
其代码如下

public class TransactionManager {
	
	private ConnectionManager connectionManager;
	
	private ThreadLocal<Transaction> transactions = new ThreadLocal<Transaction>();
	
	public Transaction beginTransaction() {
		try{
			Transaction tr = this.transactions.get();
			if( tr == null ){
				Connection connection = connectionManager.getConnection();
				connectionManager.setTransactionActive(true);
				tr = new Transaction(connection);
				this.transactions.set(tr);
			}else{
				tr.addNewService();
			}
			return tr;
		}catch(Exception e){
			throw new TransactionException();
		}
	}
	
	public void commitTransaction(Transaction transaction) {
		try{
			if( transaction.isNewTransaction() ){
				connectionManager.setTransactionActive(false);
				connectionManager.commit(transaction.getConnection());
			}
		}catch(Exception e){
			throw new TransactionException();
		}
	}
	
	public void rollbackTransaction(Transaction transaction, Exception e) {
		try{
			if( transaction.isNewTransaction() ){
				connectionManager.setTransactionActive(false);
				if(transaction.containsSavepoint()){
					Savepoint savepoint = transaction.getSavePointByException(e);
					if( savepoint == null ){
						connectionManager.rollback(transaction.getConnection());
					}else{
						connectionManager.rollback(transaction.getConnection(), savepoint);
					}
				}else{
					connectionManager.rollback(transaction.getConnection());
				}
			}
		}catch( Exception e2){
			throw new TransactionException();
		}
	}
	
	public void closeTransaction(Transaction transaction){
		try{
			if( transaction.isNewTransaction() ){
				this.transactions.remove();
				connectionManager.setTransactionActive(false);
				connectionManager.releaseConnection(transaction.getConnection());
			}else{
				transaction.completeService();
			}
		}catch(Exception e){
			throw new TransactionException();
		}
		
		
	}
	
	/**
	 * @return
	 * @throws ClassNotFoundException
	 * @throws SQLException
	 */
	

	/**
	 * @return the connectionManager
	 */
	public ConnectionManager getConnectionManager() {
		return connectionManager;
	}

	/**
	 * @param connectionManager the connectionManager to set
	 */
	public void setConnectionManager(ConnectionManager connectionManager) {
		this.connectionManager = connectionManager;
	}
	
	
	public void addSavePoint(String savePointName, Exception rollbackException){
		Transaction tr = this.transactions.get();
		if(tr == null){
			tr = this.beginTransaction();
		}
		try{
			if(!tr.isSupportSavepoint()){
				throw new SavePointNotSupportException();
			}
			tr.addSavepointAndRollbackException(savePointName, rollbackException);
		}catch(Exception e){
			throw new TransactionException();
		}
	}
	

}

5． ConnectionManager对象的实现

public class ConnectionManager {
	
	private static ConnectionManager connectionManager = new ConnectionManager();
	
	ThreadLocal<Connection> threadConnection = new ThreadLocal<Connection>();
	
	private ConnectionManager(){}
	
	public static ConnectionManager getConnectionManager(){

		return connectionManager;
	}
	
	private Boolean isTransactionActive = Boolean.FALSE;
	
	public Connection getConnection() throws ClassNotFoundException, SQLException{
		Connection connection = this.threadConnection.get();
		if( connection == null ){
			connection = (Connection) DriverManager.getConnection (
			    	"jdbc:sqlserver://10.171.30.11:1433;instanceName=nasa;databaseName=nasa2_SMS_PT;SelectMethod=cursor;characterEncoding=utf-8;autoReconnectForPools=true;autoReconnect=true", 
			    	"sms", "sms");
			this.threadConnection.set(connection);
		}
		return connection;
	}
	
	public void releaseConnection(Connection connection) throws SQLException{
		if( !isTransactionActive ){
			threadConnection.remove();
			connection.close();
		}
		
	}
	
	public void commit(Connection connection) throws SQLException{
		if( !isTransactionActive ){
			connection.commit();
		}
	}
	
	public void rollback(Connection connection) throws SQLException{
		if( !isTransactionActive ){
			connection.rollback();
		}
	}
	
	public void rollback(Connection connection, Savepoint savepoint) throws SQLException{
		if( !isTransactionActive ){
			connection.rollback(savepoint);
		}
	}
	

	/**
	 * @param isTransactionFinished the isTransactionFinished to set
	 */
	public void setTransactionActive(Boolean isTransactionActive) {
		this.isTransactionActive = isTransactionActive;
	}
	
}