setReadonly(true)
概念：从这一点设置的时间点开始到这个事务结束的过程中，其他事务所提交的数据，该事务将看不见！（查询中不会出现别人在时间点a之后提交的数据）
应用场合：
如果你一次执行单条查询语句，则没有必要启用事务支持，数据库默认支持SQL执行期间的读一致性；
如果你一次执行多条查询语句，例如统计查询，报表查询，在这种场景下，多条查询SQL必须保证整体的读一致性，否则，在前条SQL查询之后，后条SQL查询之前，数据被其他用户改变，则该次整体的统计查询将会出现读数据不一致的状态，此时，应该启用事务支持。
【注意是一次执行多次查询来统计某些信息，这时为了保证数据整体的一致性，要用只读事务】
怎样设置：
对于只读查询，可以指定事务类型为readonly，即只读事务。
由于只读事务不存在数据的修改，因此数据库将会为只读事务提供一些优化手段，例如Oracle对于只读事务，不启动回滚段，不记录回滚log。
当驱动加载完毕后，接下来就是怎样去获取一个连接
Drivermanager.getConnection(url,name,password)
跟踪代码执行到以下一步
private static Connection getConnection(
String url, java.util.Properties info, ClassLoader callerCL) throws SQLException {
java.util.Vector drivers = null;
/*
* When callerCl is null, we should check the application's
* (which is invoking this class indirectly)
* classloader, so that the JDBC driver class outside rt.jar
* can be loaded from here.
*/
synchronized(DriverManager.class) {
// synchronize loading of the correct classloader.
if(callerCL == null) {
callerCL = Thread.currentThread().getContextClassLoader();
}
}
if(url == null) {
throw new SQLException("The url cannot be null", "08001");
}
println("DriverManager.getConnection(\"" + url + "\")");
if (!initialized) {
initialize();
}
synchronized (DriverManager.class){
// use the readcopy of drivers
drivers = readDrivers;
}
// Walk through the loaded drivers attempting to make a connection.
// Remember the first exception that gets raised so we can reraise it.
SQLException reason = null;
for (int i = 0; i < drivers.size(); i++) {
DriverInfo di = (DriverInfo)drivers.elementAt(i);
// If the caller does not have permission to load the driver then
// skip it.
if ( getCallerClass(callerCL, di.driverClassName ) != di.driverClass ) {
println(" skipping: " + di);
continue;
}
try {
println(" trying " + di);
//在注册的驱动中得到相应的连接
Connection result = di.driver.connect(url, info);
if (result != null) {
// Success!
println("getConnection returning " + di);
return (result);
}
} catch (SQLException ex) {
if (reason == null) {
reason = ex;
}
}
}
// if we got here nobody could connect.
if (reason != null) {
println("getConnection failed: " + reason);
throw reason;
}
println("getConnection: no suitable driver found for "+ url);
throw new SQLException("No suitable driver found for "+ url, "08001");
}
进入com.mysql.jdbc.Driver下connect(url,info)方法NonRegisteringDriver
public java.sql.Connection connect(String url, Properties info)
throws SQLException {
if (url != null) {
if (StringUtils.startsWithIgnoreCase(url, LOADBALANCE_URL_PREFIX)) {//负载均衡的配置
return connectLoadBalanced(url, info);
} else if (StringUtils.startsWithIgnoreCase(url,
REPLICATION_URL_PREFIX)) {//复制
return connectReplicationConnection(url, info);
}
}
Properties props = null;
if ((props = parseURL(url, info)) == null) {
return null;
}
if (!"1".equals(props.getProperty(NUM_HOSTS_PROPERTY_KEY))) {
return connectFailover(url, info);
}
try {//初始化链接
Connection newConn = com.mysql.jdbc.ConnectionImpl.getInstance(
host(props), port(props), props, database(props), url);
return newConn;
} catch (SQLException sqlEx) {
// Don't wrap SQLExceptions, throw
// them un-changed.
throw sqlEx;
} catch (Exception ex) {
SQLException sqlEx = SQLError.createSQLException(Messages
.getString("NonRegisteringDriver.17") //$NON-NLS-1$
+ ex.toString()
+ Messages.getString("NonRegisteringDriver.18"), //$NON-NLS-1$
SQLError.SQL_STATE_UNABLE_TO_CONNECT_TO_DATASOURCE, null);
sqlEx.initCause(ex);
throw sqlEx;
}
}
对com.mysql.jdbc.JDBC4Connection的初始化
protected static Connection getInstance(String hostToConnectTo,
int portToConnectTo, Properties info, String databaseToConnectTo,
String url) throws SQLException {
if (!Util.isJdbc4()) {
return new ConnectionImpl(hostToConnectTo, portToConnectTo, info,
databaseToConnectTo, url);
}
//初始化
return (Connection) Util.handleNewInstance(JDBC_4_CONNECTION_CTOR,
new Object[] {
hostToConnectTo, Integer.valueOf(portToConnectTo), info,
databaseToConnectTo, url }, null);
}
进入Jdbc4Connection的代码跟踪
protected ConnectionImpl(String hostToConnectTo, int portToConnectTo, Properties info,
String databaseToConnectTo, String url)
throws SQLException {
this.charsetToNumBytesMap = new HashMap();
this.connectionCreationTimeMillis = System.currentTimeMillis();
this.pointOfOrigin = new Throwable();
if (databaseToConnectTo == null) {
databaseToConnectTo = "";
}
// Stash away for later, used to clone this connection for Statement.cancel
// and Statement.setQueryTimeout().
//
this.origHostToConnectTo = hostToConnectTo;
this.origPortToConnectTo = portToConnectTo;
this.origDatabaseToConnectTo = databaseToConnectTo;
try {
Blob.class.getMethod("truncate", new Class[] {Long.TYPE});
this.isRunningOnJDK13 = false;
} catch (NoSuchMethodException nsme) {
this.isRunningOnJDK13 = true;
}
this.sessionCalendar = new GregorianCalendar();
this.utcCalendar = new GregorianCalendar();
this.utcCalendar.setTimeZone(TimeZone.getTimeZone("GMT"));
this.log = LogFactory.getLogger(getLogger(), LOGGER_INSTANCE_NAME, getExceptionInterceptor());
// We store this per-connection, due to static synchronization
// issues in Java's built-in TimeZone class...
this.defaultTimeZone = Util.getDefaultTimeZone();
if ("GMT".equalsIgnoreCase(this.defaultTimeZone.getID())) {
this.isClientTzUTC = true;
} else {
this.isClientTzUTC = false;
}
this.openStatements = new HashMap();
if (NonRegisteringDriver.isHostPropertiesList(hostToConnectTo)) {
Properties hostSpecificProps = NonRegisteringDriver.expandHostKeyValues(hostToConnectTo);
Enumeration<?> propertyNames = hostSpecificProps.propertyNames();
while (propertyNames.hasMoreElements()) {
String propertyName = propertyNames.nextElement().toString();
String propertyValue = hostSpecificProps.getProperty(propertyName);
info.setProperty(propertyName, propertyValue);
}
} else {
if (hostToConnectTo == null) {
this.host = "localhost";
this.hostPortPair = this.host + ":" + portToConnectTo;
} else {
this.host = hostToConnectTo;
if (hostToConnectTo.indexOf(":") == -1) {
this.hostPortPair = this.host + ":" + portToConnectTo;
} else {
this.hostPortPair = this.host;
}
}
}
this.port = portToConnectTo;
this.database = databaseToConnectTo;
this.myURL = url;
this.user = info.getProperty(NonRegisteringDriver.USER_PROPERTY_KEY);
this.password = info
.getProperty(NonRegisteringDriver.PASSWORD_PROPERTY_KEY);
if ((this.user == null) || this.user.equals("")) {
this.user = "";
}
if (this.password == null) {
this.password = "";
}
this.props = info;
initializeDriverProperties(info);
try { //获取数据库的信息
this.dbmd = getMetaData(false, false);
initializeSafeStatementInterceptors();
//======================================创建远程IO连接
createNewIO(false);
unSafeStatementInterceptors();
} catch (SQLException ex) {
cleanup(ex);
// don't clobber SQL exceptions
throw ex;
} catch (Exception ex) {
cleanup(ex);
StringBuffer mesg = new StringBuffer(128);
if (!getParanoid()) {
mesg.append("Cannot connect to MySQL server on ");
mesg.append(this.host);
mesg.append(":");
mesg.append(this.port);
mesg.append(".\n\n");
mesg.append("Make sure that there is a MySQL server ");
mesg.append("running on the machine/port you are trying ");
mesg
.append("to connect to and that the machine this software is "
+ "running on ");
mesg.append("is able to connect to this host/port "
+ "(i.e. not firewalled). ");
mesg
.append("Also make sure that the server has not been started "
+ "with the --skip-networking ");
mesg.append("flag.\n\n");
} else {
mesg.append("Unable to connect to database.");
}
SQLException sqlEx = SQLError.createSQLException(mesg.toString(),
SQLError.SQL_STATE_COMMUNICATION_LINK_FAILURE, getExceptionInterceptor());
sqlEx.initCause(ex);
throw sqlEx;
}
}