## The Tomcat JDBC Connection Pool

### Table of Contents

- Introduction
- How to use
  1. Additional features
  2. Inside the Apache Tomcat Container
  3. Standalone
  4. JMX
- Attributes
  1. JNDI Factory and Type
  2. System Properties
  3. Common Attributes
  4. Tomcat JDBC Enhanced Attributes
- Advanced usage
  1. JDBC interceptors
  2. Configuring JDBC interceptors
  3. org.apache.tomcat.jdbc.pool.JdbcInterceptor
  4. org.apache.tomcat.jdbc.pool.interceptor.ConnectionState
  5. org.apache.tomcat.jdbc.pool.interceptor.StatementFinalizer
  6. org.apache.tomcat.jdbc.pool.interceptor.StatementCache
  7. org.apache.tomcat.jdbc.pool.interceptor.StatementDecoratorInterceptor
  8. org.apache.tomcat.jdbc.pool.interceptor.QueryTimeoutInterceptor
  9. org.apache.tomcat.jdbc.pool.interceptor.SlowQueryReport
  10. org.apache.tomcat.jdbc.pool.interceptor.SlowQueryReportJmx
  11. org.apache.tomcat.jdbc.pool.interceptor.ResetAbandonedTimer
- Code Example
  1. Plain Ol' Java
  2. As a Resource
  3. Asynchronous Connection Retrieval
  4. Interceptors
  5. Getting the actual JDBC connection
- Building
  1. Building from source

### Introduction

The JDBC Connection Pool `org.apache.tomcat.jdbc.pool` is a replacement or an alternative to the Apache Commons DBCP connection pool.

So why do we need a new connection pool?

Here are a few of the reasons:

1. Commons DBCP 1.x is single threaded. In order to be thread safe Commons locks the entire pool for short periods during both object allocation and object return. Note that this does not apply to Commons DBCP 2.x.
2. Commons DBCP 1.x can be slow. As the number of logical CPUs grows and the number of concurrent threads attempting to borrow or return objects increases, the performance suffers. For highly concurrent systems the impact can be significant. Note that this does not apply to Commons DBCP 2.x.
3. Commons DBCP is over 60 classes. tomcat-jdbc-pool core is 8 classes, hence modifications for future requirement will require much less changes. This is all you need to run the connection pool itself, the rest is gravy.
4. Commons DBCP uses static interfaces. This means you have to use the right version for a given JRE version or you may see `NoSuchMethodException` exceptions.
5. It's not worth rewriting over 60 classes, when a connection pool can be accomplished with a much simpler implementation.
6. Tomcat jdbc pool implements the ability retrieve a connection asynchronously, without adding additional threads to the library itself.
7. Tomcat jdbc pool is a Tomcat module, it depends on Tomcat JULI, a simplified logging framework used in Tomcat.
8. Retrieve the underlying connection using the `javax.sql.PooledConnection` interface.
9. Starvation proof. If a pool is empty, and threads are waiting for a connection, when a connection is returned, the pool will awake the correct thread waiting. Most pools will simply starve.

Features added over other connection pool implementations

1. Support for highly concurrent environments and multi core/cpu systems.
2. Dynamic implementation of interface, will support `java.sql` and `javax.sql` interfaces for your runtime environment (as long as your JDBC driver does the same), even when compiled with a lower version of the JDK.
3. Validation intervals - we don't have to validate every single time we use the connection, we can do this when we borrow or return the connection, just not more frequent than an interval we can configure.
4. Run-Once query, a configurable query that will be run only once, when the connection to the database is established. Very useful to setup session settings, that you want to exist during the entire time the connection is established.
5. Ability to configure custom interceptors. This allows you to write custom interceptors to enhance the functionality. You can use interceptors to gather query stats, cache session states, reconnect the connection upon failures, retry queries, cache query results, and so on. Your options are endless and the interceptors are dynamic, not tied to a JDK version of a `java.sql`/`javax.sql` interface.
6. High performance - we will show some differences in performance later on
7. Extremely simple, due to the very simplified implementation, the line count and source file count are very low, compare with c3p0 that has over 200 source files(last time we checked), Tomcat jdbc has a core of 8 files, the connection pool itself is about half that. As bugs may occur, they will be faster to track down, and easier to fix. Complexity reduction has been a focus from inception.
8. Asynchronous connection retrieval - you can queue your request for a connection and receive a `Future<Connection>` back.
9. Better idle connection handling. Instead of closing connections directly, it can still pool connections and sizes the idle pool with a smarter algorithm.
10. You can decide at what moment connections are considered abandoned, is it when the pool is full, or directly at a timeout by specifying a pool usage threshold.
11. The abandon connection timer will reset upon a statement/query activity. Allowing a connections that is in use for a long time to not timeout. This is achieved using the `ResetAbandonedTimer`
12. Close connections after they have been connected for a certain time. Age based close upon return to the pool.
13. Get JMX notifications and log entries when connections are suspected for being abandoned. This is similar to the `removeAbandonedTimeout` but it doesn't take any action, only reports the information. This is achieved using the `suspectTimeout` attribute.
14. Connections can be retrieved from a `java.sql.Driver`, `javax.sql.DataSource` or `javax.sql.XADataSource` This is achieved using the `dataSource` and `dataSourceJNDI` attributes.
15. XA connection support

### How to use

Usage of the Tomcat connection pool has been made to be as simple as possible, for those of you that are familiar with commons-dbcp, the transition will be very simple. Moving from other connection pools is also fairly straight forward.

#### Additional features

The Tomcat connection pool offers a few additional features over what most other pools let you do:

- `initSQL` - the ability to run an SQL statement exactly once, when the connection is created
- `validationInterval` - in addition to running validations on connections, avoid running them too frequently.
- `jdbcInterceptors` - flexible and pluggable interceptors to create any customizations around the pool, the query execution and the result set handling. More on this in the advanced section.
- `fairQueue` - Set the fair flag to true to achieve thread fairness or to use asynchronous connection retrieval

#### Inside the Apache Tomcat Container

The Tomcat Connection pool is configured as a resource described in The Tomcat JDBC documentation With the only difference being that you have to specify the `factory` attribute and set the value to `org.apache.tomcat.jdbc.pool.DataSourceFactory`

#### Standalone

The connection pool only has another dependency, and that is on tomcat-juli.jar. To configure the pool in a stand alone project using bean instantiation, the bean to instantiate is `org.apache.tomcat.jdbc.pool.DataSource`. The same attributes (documented below) as you use to configure a connection pool as a JNDI resource, are used to configure a data source as a bean.

#### JMX

The connection pool object exposes an MBean that can be registered. In order for the connection pool object to create the MBean, the flag `jmxEnabled` has to be set to true. This doesn't imply that the pool will be registered with an MBean server, merely that the MBean is created. In a container like Tomcat, Tomcat itself registers the DataSource with the MBean server, the `org.apache.tomcat.jdbc.pool.DataSource` object will then register the actual connection pool MBean. If you're running outside of a container, you can register the DataSource yourself under any object name you specify, and it propagates the registration to the underlying pool. To do this you would call `mBeanServer.registerMBean(dataSource.getPool().getJmxPool(),objectname)`. Prior to this call, ensure that the pool has been created by calling `dataSource.createPool()`.

### Attributes

To provide a very simple switch to and from commons-dbcp and tomcat-jdbc-pool, Most attributes are the same and have the same meaning.

#### JNDI Factory and Type

Attribute | Description
--- | ---
`factory` | factory is required, and the value should be `org.apache.tomcat.jdbc.pool.DataSourceFactory`
`type` | Type should always be `javax.sql.DataSource` or `javax.sql.XADataSource` Depending on the type a `org.apache.tomcat.jdbc.pool.DataSource` or a `org.apache.tomcat.jdbc.pool.XADataSource` will be created.

#### System Properties

System properties are JVM wide, affect all pools created in the JVM

Attribute | Description
--- | ---
`org.apache.tomcat.jdbc.pool.onlyAttemptCurrentClassLoader` | (boolean) Controls classloading of dynamic classes, such as JDBC drivers, interceptors and validators. If set to `false`, default value, the pool will first attempt to load using the current loader (i.e. the class loader that loaded the pool classes) and if class loading fails attempt to load using the thread context loader. Set this value to `true`, if you wish to remain backwards compatible with Apache Tomcat 8.0.8 and earlier, and only attempt the current loader. If not set then the default value is `false`.

#### Common Attributes

These attributes are shared between commons-dbcp and tomcat-jdbc-pool, in some cases default values are different.

Attribute | Description
--- | ---
`defaultAutoCommit` | (boolean) The default auto-commit state of connections created by this pool. If not set, default is JDBC driver default (If not set then the `setAutoCommit` method will not be called.)
`defaultReadOnly` | (boolean) The default read-only state of connections created by this pool. If not set then the `setReadOnly` method will not be called. (Some drivers don't support read only mode, ex: Informix)
`defaultTransactionIsolation` | (String) The default TransactionIsolation state of connections created by this pool. One of the following: (see javadoc ) `NONE` `READ_COMMITTED` `READ_UNCOMMITTED` `REPEATABLE_READ` `SERIALIZABLE` If not set, the method will not be called and it defaults to the JDBC driver.
`defaultCatalog` | (String) The default catalog of connections created by this pool.
`driverClassName` | (String) The fully qualified Java class name of the JDBC driver to be used. The driver has to be accessible from the same classloader as tomcat-jdbc.jar
`username` | (String) The connection username to be passed to our JDBC driver to establish a connection. Note that method `DataSource.getConnection(username,password)` by default will not use credentials passed into the method, but will use the ones configured here. See `alternateUsernameAllowed` property for more details.
`password` | (String) The connection password to be passed to our JDBC driver to establish a connection. Note that method `DataSource.getConnection(username,password)` by default will not use credentials passed into the method, but will use the ones configured here. See `alternateUsernameAllowed` property for more details.
`maxActive` | (int) The maximum number of active connections that can be allocated from this pool at the same time. The default value is `100`
`maxIdle` | (int) The maximum number of connections that should be kept in the pool at all times. Default value is `maxActive`:`100` Idle connections are checked periodically (if enabled) and connections that been idle for longer than `minEvictableIdleTimeMillis` will be released. (also see `testWhileIdle`)
`minIdle` | (int) The minimum number of established connections that should be kept in the pool at all times. The connection pool can shrink below this number if validation queries fail. Default value is derived from `initialSize`:`10` (also see `testWhileIdle`)
`initialSize` | (int)The initial number of connections that are created when the pool is started. Default value is `10`
`maxWait` | (int) The maximum number of milliseconds that the pool will wait (when there are no available connections) for a connection to be returned before throwing an exception. Default value is `30000` (30 seconds)
`testOnBorrow` | (boolean) The indication of whether objects will be validated before being borrowed from the pool. If the object fails to validate, it will be dropped from the pool, and we will attempt to borrow another. In order to have a more efficient validation, see `validationInterval`. Default value is `false`
`testOnConnect` | (boolean) The indication of whether objects will be validated when a connection is first created. If an object fails to validate, it will be throw `SQLException`. Default value is `false`
`testOnReturn` | (boolean) The indication of whether objects will be validated before being returned to the pool. The default value is `false`.
`testWhileIdle` | (boolean) The indication of whether objects will be validated by the idle object evictor (if any). If an object fails to validate, it will be dropped from the pool. The default value is `false` and this property has to be set in order for the pool cleaner/test thread is to run (also see `timeBetweenEvictionRunsMillis`)
`validationQuery` | (String) The SQL query that will be used to validate connections from this pool before returning them to the caller. If specified, this query does not have to return any data, it just can't throw a `SQLException`. The default value is `null`. If not specified, connections will be validation by the isValid() method. Example values are `SELECT 1`(mysql), `select 1 from dual`(oracle), `SELECT 1`(MS Sql Server)
`validationQueryTimeout` | (int) The timeout in seconds before a connection validation queries fail. This works by calling `java.sql.Statement.setQueryTimeout(seconds)` on the statement that executes the `validationQuery`. The pool itself doesn't timeout the query, it is still up to the JDBC driver to enforce query timeouts. A value less than or equal to zero will disable this feature. The default value is `-1`.
`validatorClassName` | (String) The name of a class which implements the `org.apache.tomcat.jdbc.pool.Validator` interface and provides a no-arg constructor (may be implicit). If specified, the class will be used to create a Validator instance which is then used instead of any validation query to validate connections. The default value is `null`. An example value is `com.mycompany.project.SimpleValidator`.
`timeBetweenEvictionRunsMillis` | (int) The number of milliseconds to sleep between runs of the idle connection validation/cleaner thread. This value should not be set under 1 second. It dictates how often we check for idle, abandoned connections, and how often we validate idle connections. The default value is `5000` (5 seconds).
`numTestsPerEvictionRun` | (int) Property not used in tomcat-jdbc-pool.
`minEvictableIdleTimeMillis` | (int) The minimum amount of time an object may sit idle in the pool before it is eligible for eviction. The default value is `60000` (60 seconds).
`accessToUnderlyingConnectionAllowed` | (boolean) Property not used. Access can be achieved by calling `unwrap` on the pooled connection. see `javax.sql.DataSource` interface, or call `getConnection` through reflection or cast the object as `javax.sql.PooledConnection`
`removeAbandoned` | (boolean) Flag to remove abandoned connections if they exceed the `removeAbandonedTimeout`. If set to true a connection is considered abandoned and eligible for removal if it has been in use longer than the `removeAbandonedTimeout` Setting this to `true` can recover db connections from applications that fail to close a connection. See also `logAbandoned` The default value is `false`.
`removeAbandonedTimeout` | (int) Timeout in seconds before an abandoned(in use) connection can be removed. The default value is `60` (60 seconds). The value should be set to the longest running query your applications might have.
`logAbandoned` | (boolean) Flag to log stack traces for application code which abandoned a Connection. Logging of abandoned Connections adds overhead for every Connection borrow because a stack trace has to be generated. The default value is `false`.
`connectionProperties` | (String) The connection properties that will be sent to our JDBC driver when establishing new connections. Format of the string must be [propertyName=property;]* NOTE - The "user" and "password" properties will be passed explicitly, so they do not need to be included here. The default value is `null`.
`poolPreparedStatements` | (boolean) Property not used.
`maxOpenPreparedStatements` | (int) Property not used.

#### Tomcat JDBC Enhanced Attributes

Attribute | Description
--- | ---
`initSQL` | (String) A custom query to be run when a connection is first created. The default value is `null`.
`jdbcInterceptors` | (String) A semicolon separated list of classnames extending `org.apache.tomcat.jdbc.pool.JdbcInterceptor` class. See Configuring JDBC interceptors below for more detailed description of syntax and examples. These interceptors will be inserted as an interceptor into the chain of operations on a `java.sql.Connection` object. The default value is `null`. Predefined interceptors: `org.apache.tomcat.jdbc.pool.interceptor. ConnectionState` - keeps track of auto commit, read only, catalog and transaction isolation level. `org.apache.tomcat.jdbc.pool.interceptor. StatementFinalizer` - keeps track of opened statements, and closes them when the connection is returned to the pool. More predefined interceptors are described in detail in the JDBC Interceptors section.
`validationInterval` | (long) avoid excess validation, only run validation at most at this frequency - time in milliseconds. If a connection is due for validation, but has been validated previously within this interval, it will not be validated again. The default value is `3000` (3 seconds).
`jmxEnabled` | (boolean) Register the pool with JMX or not. The default value is `true`.
`fairQueue` | (boolean) Set to true if you wish that calls to getConnection should be treated fairly in a true FIFO fashion. This uses the `org.apache.tomcat.jdbc.pool.FairBlockingQueue` implementation for the list of the idle connections. The default value is `true`. This flag is required when you want to use asynchronous connection retrieval. Setting this flag ensures that threads receive connections in the order they arrive. During performance tests, there is a very large difference in how locks and lock waiting is implemented. When `fairQueue=true` there is a decision making process based on what operating system the system is running. If the system is running on Linux (property `os.name=Linux`. To disable this Linux specific behavior and still use the fair queue, simply add the property `org.apache.tomcat.jdbc.pool.FairBlockingQueue.ignoreOS=true` to your system properties before the connection pool classes are loaded.
`abandonWhenPercentageFull` | (int) Connections that have been abandoned (timed out) won't get closed and reported up unless the number of connections in use are above the percentage defined by `abandonWhenPercentageFull`. The value should be between 0-100. The default value is `0`, which implies that connections are eligible for closure as soon as `removeAbandonedTimeout` has been reached.
`maxAge` | (long) Time in milliseconds to keep this connection. This attribute works both when returning connection and when borrowing connection. When a connection is borrowed from the pool, the pool will check to see if the `now - time-when-connected > maxAge` has been reached , and if so, it reconnects before borrow it. When a connection is returned to the pool, the pool will check to see if the `now - time-when-connected > maxAge` has been reached, and if so, it closes the connection rather than returning it to the pool. The default value is `0`, which implies that connections will be left open and no age check will be done upon borrowing from the pool and returning the connection to the pool.
`useEquals` | (boolean) Set to true if you wish the `ProxyConnection` class to use `String.equals` and set to `false` when you wish to use `==` when comparing method names. This property does not apply to added interceptors as those are configured individually. The default value is `true`.
`suspectTimeout` | (int) Timeout value in seconds. Default value is `0`. Similar to to the `removeAbandonedTimeout` value but instead of treating the connection as abandoned, and potentially closing the connection, this simply logs the warning if `logAbandoned` is set to true. If this value is equal or less than 0, no suspect checking will be performed. Suspect checking only takes place if the timeout value is larger than 0 and the connection was not abandoned or if abandon check is disabled. If a connection is suspect a WARN message gets logged and a JMX notification gets sent once.
`rollbackOnReturn` | (boolean) If `autoCommit==false` then the pool can terminate the transaction by calling rollback on the connection as it is returned to the pool Default value is `false`.
`commitOnReturn` | (boolean) If `autoCommit==false` then the pool can complete the transaction by calling commit on the connection as it is returned to the pool If `rollbackOnReturn==true` then this attribute is ignored. Default value is `false`.
`alternateUsernameAllowed` | (boolean) By default, the jdbc-pool will ignore the `DataSource.getConnection(username,password)` call, and simply return a previously pooled connection under the globally configured properties `username` and `password`, for performance reasons. The pool can however be configured to allow use of different credentials each time a connection is requested. To enable the functionality described in the `DataSource.getConnection(username,password)` call, simply set the property `alternateUsernameAllowed` to `true`. Should you request a connection with the credentials user1/password1 and the connection was previously connected using different user2/password2, the connection will be closed, and reopened with the requested credentials. This way, the pool size is still managed on a global level, and not on a per schema level. The default value is `false`. This property was added as an enhancement to bug 50025.
`dataSource` | (javax.sql.DataSource) Inject a data source to the connection pool, and the pool will use the data source to retrieve connections instead of establishing them using the `java.sql.Driver` interface. This is useful when you wish to pool XA connections or connections established using a data source instead of a connection string. Default value is `null`
`dataSourceJNDI` | (String) The JNDI name for a data source to be looked up in JNDI and then used to establish connections to the database. See the `dataSource` attribute. Default value is `null`
`useDisposableConnectionFacade` | (boolean) Set this to true if you wish to put a facade on your connection so that it cannot be reused after it has been closed. This prevents a thread holding on to a reference of a connection it has already called closed on, to execute queries on it. Default value is `true`.
`logValidationErrors` | (boolean) Set this to true to log errors during the validation phase to the log file. If set to true, errors will be logged as SEVERE. Default value is `false` for backwards compatibility.
`propagateInterruptState` | (boolean) Set this to true to propagate the interrupt state for a thread that has been interrupted (not clearing the interrupt state). Default value is `false` for backwards compatibility.
`ignoreExceptionOnPreLoad` | (boolean) Flag whether ignore error of connection creation while initializing the pool. Set to true if you want to ignore error of connection creation while initializing the pool. Set to false if you want to fail the initialization of the pool by throwing exception. The default value is `false`.
`useStatementFacade` | (boolean) Set this to true if you wish to wrap statements in order to enable `equals()` and `hashCode()` methods to be called on the closed statements if any statement proxy is set. Default value is `true`.

### Advanced usage

#### JDBC interceptors

To see an example of how to use an interceptor, take a look at `org.apache.tomcat.jdbc.pool.interceptor.ConnectionState`. This simple interceptor is a cache of three attributes, transaction isolation level, auto commit and read only state, in order for the system to avoid not needed roundtrips to the database.

Further interceptors will be added to the core of the pool as the need arises. Contributions are always welcome!

Interceptors are of course not limited to just `java.sql.Connection` but can be used to wrap any of the results from a method invocation as well. You could build query performance analyzer that provides JMX notifications when a query is running longer than the expected time.

#### Configuring JDBC interceptors

Configuring JDBC interceptors is done using the jdbcInterceptors property. The property contains a list of semicolon separated class names. If the classname is not fully qualified it will be prefixed with the `org.apache.tomcat.jdbc.pool.interceptor.` prefix.

Example:
`jdbcInterceptors="org.apache.tomcat.jdbc.pool.interceptor.ConnectionState; org.apache.tomcat.jdbc.pool.interceptor.StatementFinalizer"`
is the same as
`jdbcInterceptors="ConnectionState;StatementFinalizer"`

Interceptors can have properties as well. Properties for an interceptor are specified within parentheses after the class name. Several properties are separated by commas.

Example:
`jdbcInterceptors="ConnectionState;StatementFinalizer(useEquals=true)"`

Extra whitespace characters around class names, property names and values are ignored.

#### org.apache.tomcat.jdbc.pool.JdbcInterceptor

Abstract base class for all interceptors, cannot be instantiated.

Attribute | Description
--- | ---
`useEquals` | (boolean) Set to true if you wish the `ProxyConnection` class to use `String.equals` and set to `false` when you wish to use `==` when comparing method names. The default value is `true`.

#### org.apache.tomcat.jdbc.pool.interceptor.ConnectionState

Caches the connection for the following attributes `autoCommit`, `readOnly`, `transactionIsolation` and `catalog`. It is a performance enhancement to avoid roundtrip to the database when getters are called or setters are called with an already set value.

Attribute | Description
--- | ---

#### org.apache.tomcat.jdbc.pool.interceptor.StatementFinalizer

Keeps track of all statements created using `createStatement`, `prepareStatement` or `prepareCall` and closes these statements when the connection is returned to the pool.

Attribute | Description
--- | ---
`trace` | (boolean as String) Enable tracing of unclosed statements. When enabled and a connection is closed, and statements are not closed, the interceptor will log all stack traces. The default value is `false`.

#### org.apache.tomcat.jdbc.pool.interceptor.StatementCache

Caches `PreparedStatement` and/or `CallableStatement` instances on a connection.

The statements are cached per connection. The count limit is counted globally for all connections that belong to the same pool. Once the count reaches `max`, subsequent statements are not returned to the cache and are closed immediately.

Attribute | Description
--- | ---
`prepared` | (boolean as String) Enable caching of `PreparedStatement` instances created using `prepareStatement` calls. The default value is `true`.
`callable` | (boolean as String) Enable caching of `CallableStatement` instances created using `prepareCall` calls. The default value is `false`.
`max` | (int as String) Limit on the count of cached statements across the connection pool. The default value is `50`.

#### org.apache.tomcat.jdbc.pool.interceptor.StatementDecoratorInterceptor

See 48392. Interceptor to wrap statements and result sets in order to prevent access to the actual connection using the methods `ResultSet.getStatement().getConnection()` and `Statement.getConnection()`

Attribute | Description
--- | ---

#### org.apache.tomcat.jdbc.pool.interceptor.QueryTimeoutInterceptor

Automatically calls `java.sql.Statement.setQueryTimeout(seconds)` when a new statement is created. The pool itself doesn't timeout the query, it is still up to the JDBC driver to enforce query timeouts.

Attribute | Description
--- | ---
`queryTimeout` | (int as String) The number of seconds to set for the query timeout. A value less than or equal to zero will disable this feature. The default value is `1` seconds.

#### org.apache.tomcat.jdbc.pool.interceptor.SlowQueryReport

Keeps track of query performance and issues log entries when queries exceed a time threshold of fail. The log level used is `WARN`

Attribute | Description
--- | ---
`threshold` | (int as String) The number of milliseconds a query has to exceed before issuing a log alert. The default value is `1000` milliseconds.
`maxQueries` | (int as String) The maximum number of queries to keep track of in order to preserve memory space. A value less than or equal to 0 will disable this feature. The default value is `1000`.
`logSlow` | (boolean as String) Set to `true` if you wish to log slow queries. The default value is `true`.
`logFailed` | (boolean as String) Set to `true` if you wish to log failed queries. The default value is `false`.

#### org.apache.tomcat.jdbc.pool.interceptor.SlowQueryReportJmx

Extends the `SlowQueryReport` and in addition to log entries it issues JMX notification for monitoring tools to react to. Inherits all the attributes from its parent class. This class uses Tomcat's JMX engine so it won't work outside of the Tomcat container. By default, JMX notifications are sent through the ConnectionPool mbean if it is enabled. The `SlowQueryReportJmx` can also register an MBean if `notifyPool=false`

Attribute | Description
--- | ---
`notifyPool` | (boolean as String) Set to false if you want JMX notifications to go to the `SlowQueryReportJmx` MBean The default value is `true`.
`objectName` | (String) Define a valid `javax.management.ObjectName` string that will be used to register this object with the platform mbean server The default value is `null` and the object will be registered using tomcat.jdbc:type=org.apache.tomcat.jdbc.pool.interceptor.SlowQueryReportJmx,name=the-name-of-the-pool

#### org.apache.tomcat.jdbc.pool.interceptor.ResetAbandonedTimer

The abandoned timer starts when a connection is checked out from the pool. This means if you have a 30second timeout and run 10x10second queries using the connection it will be marked abandoned and potentially reclaimed depending on the `abandonWhenPercentageFull` attribute. Using this interceptor it will reset the checkout timer every time you perform an operation on the connection or execute a query successfully.

Attribute | Description
--- | ---

### Code Example

Other examples of Tomcat configuration for JDBC usage can be found in the Tomcat documentation.

#### Plain Ol' Java

Here is a simple example of how to create and use a data source.

      import java.sql.Connection;
      import java.sql.ResultSet;
      import java.sql.Statement;

      import org.apache.tomcat.jdbc.pool.DataSource;
      import org.apache.tomcat.jdbc.pool.PoolProperties;

      public class SimplePOJOExample {

          public static void main(String[] args) throws Exception {
              PoolProperties p = new PoolProperties();
              p.setUrl("jdbc:mysql://localhost:3306/mysql");
              p.setDriverClassName("com.mysql.jdbc.Driver");
              p.setUsername("root");
              p.setPassword("password");
              p.setJmxEnabled(true);
              p.setTestWhileIdle(false);
              p.setTestOnBorrow(true);
              p.setValidationQuery("SELECT 1");
              p.setTestOnReturn(false);
              p.setValidationInterval(30000);
              p.setTimeBetweenEvictionRunsMillis(30000);
              p.setMaxActive(100);
              p.setInitialSize(10);
              p.setMaxWait(10000);
              p.setRemoveAbandonedTimeout(60);
              p.setMinEvictableIdleTimeMillis(30000);
              p.setMinIdle(10);
              p.setLogAbandoned(true);
              p.setRemoveAbandoned(true);
              p.setJdbcInterceptors(
                "org.apache.tomcat.jdbc.pool.interceptor.ConnectionState;"+
                "org.apache.tomcat.jdbc.pool.interceptor.StatementFinalizer");
              DataSource datasource = new DataSource();
              datasource.setPoolProperties(p);

              Connection con = null;
              try {
                con = datasource.getConnection();
                Statement st = con.createStatement();
                ResultSet rs = st.executeQuery("select * from user");
                int cnt = 1;
                while (rs.next()) {
                    System.out.println((cnt++)+". Host:" +rs.getString("Host")+
                      " User:"+rs.getString("User")+" Password:"+rs.getString("Password"));
                }
                rs.close();
                st.close();
              } finally {
                if (con!=null) try {con.close();}catch (Exception ignore) {}
              }
          }

      }

#### As a Resource

And here is an example on how to configure a resource for JNDI lookups

    <Resource name="jdbc/TestDB"
              auth="Container"
              type="javax.sql.DataSource"
              factory="org.apache.tomcat.jdbc.pool.DataSourceFactory"
              testWhileIdle="true"
              testOnBorrow="true"
              testOnReturn="false"
              validationQuery="SELECT 1"
              validationInterval="30000"
              timeBetweenEvictionRunsMillis="30000"
              maxActive="100"
              minIdle="10"
              maxWait="10000"
              initialSize="10"
              removeAbandonedTimeout="60"
              removeAbandoned="true"
              logAbandoned="true"
              minEvictableIdleTimeMillis="30000"
              jmxEnabled="true"
              jdbcInterceptors="org.apache.tomcat.jdbc.pool.interceptor.ConnectionState;
                org.apache.tomcat.jdbc.pool.interceptor.StatementFinalizer"
              username="root"
              password="password"
              driverClassName="com.mysql.jdbc.Driver"
              url="jdbc:mysql://localhost:3306/mysql"/>

#### Asynchronous Connection Retrieval

The Tomcat JDBC connection pool supports asynchronous connection retrieval without adding additional threads to the pool library. It does this by adding a method to the data source called `Future<Connection> getConnectionAsync()`. In order to use the async retrieval, two conditions must be met:

1. You must configure the `fairQueue` property to be `true`.
2. You will have to cast the data source to `org.apache.tomcat.jdbc.pool.DataSource`

An example of using the async feature is show below.

      Connection con = null;
      try {
        Future<Connection> future = datasource.getConnectionAsync();
        while (!future.isDone()) {
          System.out.println("Connection is not yet available. Do some background work");
          try {
            Thread.sleep(100); //simulate work
          }catch (InterruptedException x) {
            Thread.currentThread().interrupt();
          }
        }
        con = future.get(); //should return instantly
        Statement st = con.createStatement();
        ResultSet rs = st.executeQuery("select * from user");

#### Interceptors

Interceptors are a powerful way to enable, disable or modify functionality on a specific connection or its sub components. There are many different use cases for when interceptors are useful. By default, and for performance reasons, the connection pool is stateless. The only state the pool itself inserts are `defaultAutoCommit`, `defaultReadOnly`, `defaultTransactionIsolation`, `defaultCatalog` if these are set. These 4 properties are only set upon connection creation. Should these properties be modified during the usage of the connection, the pool itself will not reset them.

An interceptor has to extend the `org.apache.tomcat.jdbc.pool.JdbcInterceptor` class. This class is fairly simple, You will need to have a no arg constructor

      public JdbcInterceptor() {
      }

When a connection is borrowed from the pool, the interceptor can initialize or in some other way react to the event by implementing the

      public abstract void reset(ConnectionPool parent, PooledConnection con);

method. This method gets called with two parameters, a reference to the connection pool itself `ConnectionPool parent` and a reference to the underlying connection `PooledConnection con`.

When a method on the `java.sql.Connection` object is invoked, it will cause the

      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable

method to get invoked. The `Method method` is the actual method invoked, and `Object[] args` are the arguments. To look at a very simple example, where we demonstrate how to make the invocation to `java.sql.Connection.close()` a noop if the connection has been closed

      if (CLOSE_VAL==method.getName()) {
          if (isClosed()) return null; //noop for already closed.
      }
      return super.invoke(proxy,method,args);

There is an observation being made. It is the comparison of the method name. One way to do this would be to do `"close".equals(method.getName())`. Above we see a direct reference comparison between the method name and `static final String` reference. According to the JVM spec, method names and static final String end up in a shared constant pool, so the reference comparison should work. One could of course do this as well:

      if (compare(CLOSE_VAL,method)) {
          if (isClosed()) return null; //noop for already closed.
      }
      return super.invoke(proxy,method,args);

The `compare(String,Method)` will use the `useEquals` flag on an interceptor and do either reference comparison or a string value comparison when the `useEquals=true` flag is set.

Pool start/stop
When the connection pool is started or closed, you can be notified. You will only be notified once per interceptor class even though it is an instance method. and you will be notified using an interceptor currently not attached to a pool.

      public void poolStarted(ConnectionPool pool) {
      }

      public void poolClosed(ConnectionPool pool) {
      }

When overriding these methods, don't forget to call super if you are extending a class other than `JdbcInterceptor`

Configuring interceptors
Interceptors are configured using the `jdbcInterceptors` property or the `setJdbcInterceptors` method. An interceptor can have properties, and would be configured like this

      String jdbcInterceptors=
        "org.apache.tomcat.jdbc.pool.interceptor.ConnectionState(useEquals=true,fast=yes)"

Interceptor properties
Since interceptors can have properties, you need to be able to read the values of these properties within your interceptor. Taking an example like the one above, you can override the `setProperties` method.

      public void setProperties(Map<String, InterceptorProperty> properties) {
         super.setProperties(properties);
         final String myprop = "myprop";
         InterceptorProperty p1 = properties.get(myprop);
         if (p1!=null) {
             setMyprop(Long.parseLong(p1.getValue()));
         }
      }

#### Getting the actual JDBC connection

Connection pools create wrappers around the actual connection in order to properly pool them. We also create interceptors in these wrappers to be able to perform certain functions. If there is a need to retrieve the actual connection, one can do so using the `javax.sql.PooledConnection` interface.

      Connection con = datasource.getConnection();
      Connection actual = ((javax.sql.PooledConnection)con).getConnection();

### Building

We build the JDBC pool code with 1.6, but it is backwards compatible down to 1.5 for runtime environment. For unit test, we use 1.6 and higher

Other examples of Tomcat configuration for JDBC usage can be found in the Tomcat documentation.

#### Building from source

Building is pretty simple. The pool has a dependency on `tomcat-juli.jar` and in case you want the `SlowQueryReportJmx`

      javac -classpath tomcat-juli.jar \
            -d . \
            org/apache/tomcat/jdbc/pool/*.java \
            org/apache/tomcat/jdbc/pool/interceptor/*.java \
            org/apache/tomcat/jdbc/pool/jmx/*.java

A build file can be found in the Tomcat source repository.

As a convenience, a build file is also included where a simple build command will generate all files needed.

      ant download  (downloads dependencies)
      ant build     (compiles and generates .jar files)
      ant dist      (creates a release package)
      ant test      (runs tests, expects a test database to be setup)

The system is structured for a Maven build, but does generate release artifacts. Just the library itself.