Apache Tomcat 10 (10.1.39) - JNDI Resources How-To
Author: Craig R. McClanahan

## JNDI Resources How-To

### Table of Contents

- Introduction
- web.xml configuration
- context.xml configuration
- Global configuration
- Using resources
- Tomcat Standard Resource Factories
  1. Generic JavaBean Resources
  2. Memory UserDatabase Resources
  3. DataSource UserDatabase Resources
  4. Jakarta Mail Sessions
  5. JDBC Data Sources
- Adding Custom Resource Factories

### Introduction

Tomcat provides a JNDI InitialContext implementation instance for each web application running under it, in a manner that is compatible with those provided by a Jakarta EE application server. The Jakarta EE standard provides a standard set of elements in the `/WEB-INF/web.xml` file to reference/define resources.

See the following Specifications for more information about programming APIs for JNDI, and for the features supported by Jakarta EE servers, which Tomcat emulates for the services that it provides:

- Java Naming and Directory Interface (included in JDK 1.4 onwards)
- Jakarta EE Platform Specification (in particular, see Chapter 5 on Naming)

### web.xml configuration

The following elements may be used in the web application deployment descriptor (`/WEB-INF/web.xml`) of your web application to define resources:

- `<env-entry>` - Environment entry, a single-value parameter that can be used to configure how the application will operate.
- `<resource-ref>` - Resource reference, which is typically to an object factory for resources such as a JDBC `DataSource`, a Jakarta Mail `Session`, or custom object factories configured into Tomcat.
- `<resource-env-ref>` - Resource environment reference, a new variation of `resource-ref` added in Servlet 2.4 that is simpler to configure for resources that do not require authentication information.

Providing that Tomcat is able to identify an appropriate resource factory to use to create the resource and that no further configuration information is required, Tomcat will use the information in `/WEB-INF/web.xml` to create the resource.

Tomcat provides a number of Tomcat specific options for JNDI resources that cannot be specified in web.xml. These include `closeMethod` that enables faster cleaning-up of JNDI resources when a web application stops and `singleton` that controls whether or not a new instance of the resource is created for every JNDI lookup. To use these configuration options the resource must be specified in a web application's `<Context>` element or in the `<GlobalNamingResources>` element of `$CATALINA_BASE/conf/server.xml`.

### context.xml configuration

If Tomcat is unable to identify the appropriate resource factory and/or additional configuration information is required, additional Tomcat specific configuration must be specified before Tomcat can create the resource. Tomcat specific resource configuration is entered in the `<Context>` elements that can be specified in either `$CATALINA_BASE/conf/server.xml` or, preferably, the per-web-application context XML file (`META-INF/context.xml`).

Tomcat specific resource configuration is performed using the following elements in the `<Context>` element:

- <Environment> - Configure names and values for scalar environment entries that will be exposed to the web application through the JNDI `InitialContext` (equivalent to the inclusion of an `<env-entry>` element in the web application deployment descriptor).
- <Resource> - Configure the name and data type of a resource made available to the application (equivalent to the inclusion of a `<resource-ref>` element in the web application deployment descriptor).
- <ResourceLink> - Add a link to a resource defined in the global JNDI context. Use resource links to give a web application access to a resource defined in the <GlobalNamingResources> child element of the <Server> element.
- <Transaction> - Add a resource factory for instantiating the UserTransaction object instance that is available at `java:comp/UserTransaction`.

Any number of these elements may be nested inside a `<Context>` element and will be associated only with that particular web application.

If a resource has been defined in a `<Context>` element it is not necessary for that resource to be defined in `/WEB-INF/web.xml`. However, it is recommended to keep the entry in `/WEB-INF/web.xml` to document the resource requirements for the web application.

Where the same resource name has been defined for a `<env-entry>` element included in the web application deployment descriptor (`/WEB-INF/web.xml`) and in an `<Environment>` element as part of the `<Context>` element for the web application, the values in the deployment descriptor will take precedence only if allowed by the corresponding `<Environment>` element (by setting the `override` attribute to "true").

### Global configuration

Tomcat maintains a separate namespace of global resources for the entire server. These are configured in the `<GlobalNamingResources>` element of `$CATALINA_BASE/conf/server.xml`. You may expose these resources to web applications by using a <ResourceLink> to include it in the per-web-application context.

If a resource has been defined using a <ResourceLink>, it is not necessary for that resource to be defined in `/WEB-INF/web.xml`. However, it is recommended to keep the entry in `/WEB-INF/web.xml` to document the resource requirements for the web application.

### Using resources

The `InitialContext` is configured as a web application is initially deployed, and is made available to web application components (for read-only access). All configured entries and resources are placed in the `java:comp/env` portion of the JNDI namespace, so a typical access to a resource - in this case, to a JDBC `DataSource` - would look something like this:

    // Obtain our environment naming context
    Context initCtx = new InitialContext();
    Context envCtx = (Context) initCtx.lookup("java:comp/env");

    // Look up our data source
    DataSource ds = (DataSource)
      envCtx.lookup("jdbc/EmployeeDB");

    // Allocate and use a connection from the pool
    Connection conn = ds.getConnection();
    ... use this connection to access the database ...
    conn.close();

### Tomcat Standard Resource Factories

Tomcat includes a series of standard resource factories that can provide services to your web applications, but give you configuration flexibility (via the `<Context>` element) without modifying the web application or the deployment descriptor. Each subsection below details the configuration and usage of the standard resource factories.

See Adding Custom Resource Factories for information about how to create, install, configure, and use your own custom resource factory classes with Tomcat.

NOTE - Of the standard resource factories, only the "JDBC Data Source" and "User Transaction" factories are mandated to be available on other platforms, and then they are required only if the platform implements the Jakarta EE specs. All other standard resource factories, plus custom resource factories that you write yourself, are specific to Tomcat and cannot be assumed to be available on other containers.

#### Generic JavaBean Resources

##### 0. Introduction

This resource factory can be used to create objects of any Java class that conforms to standard JavaBeans naming conventions (i.e. it has a zero-arguments constructor, and has property setters that conform to the setFoo() naming pattern. The resource factory will only create a new instance of the appropriate bean class every time a `lookup()` for this entry is made if the `singleton` attribute of the factory is set to `false`.

The steps required to use this facility are described below.

##### 1. Create Your JavaBean Class

Create the JavaBean class which will be instantiated each time that the resource factory is looked up. For this example, assume you create a class `com.mycompany.MyBean`, which looks like this:

    package com.mycompany;

    public class MyBean {

      private String foo = "Default Foo";

      public String getFoo() {
        return (this.foo);
      }

      public void setFoo(String foo) {
        this.foo = foo;
      }

      private int bar = 0;

      public int getBar() {
        return (this.bar);
      }

      public void setBar(int bar) {
        this.bar = bar;
      }

    }

##### 2. Declare Your Resource Requirements

Next, modify your web application deployment descriptor (`/WEB-INF/web.xml`) to declare the JNDI name under which you will request new instances of this bean. The simplest approach is to use a `<resource-env-ref>` element, like this:

    <resource-env-ref>
      <description>
        Object factory for MyBean instances.
      </description>
      <resource-env-ref-name>
        bean/MyBeanFactory
      </resource-env-ref-name>
      <resource-env-ref-type>
        com.mycompany.MyBean
      </resource-env-ref-type>
    </resource-env-ref>

WARNING - Be sure you respect the element ordering that is required by the DTD for web application deployment descriptors! See the Servlet Specification for details.

##### 3. Code Your Application's Use Of This Resource

A typical use of this resource environment reference might look like this:

    Context initCtx = new InitialContext();
    Context envCtx = (Context) initCtx.lookup("java:comp/env");
    MyBean bean = (MyBean) envCtx.lookup("bean/MyBeanFactory");

    writer.println("foo = " + bean.getFoo() + ", bar = " +
                   bean.getBar());

##### 4. Configure Tomcat's Resource Factory

To configure Tomcat's resource factory, add an element like this to the `<Context>` element for this web application.

    <Context ...>
      ...
      <Resource name="bean/MyBeanFactory" auth="Container"
                type="com.mycompany.MyBean"
                factory="org.apache.naming.factory.BeanFactory"
                bar="23"/>
      ...
    </Context>

Note that the resource name (here, `bean/MyBeanFactory` must match the value specified in the web application deployment descriptor. We are also initializing the value of the `bar` property, which will cause `setBar(23)` to be called before the new bean is returned. Because we are not initializing the `foo` property (although we could have), the bean will contain whatever default value is set up by its constructor.

If the bean property is of type `String`, the BeanFactory will call the property setter using the provided property value. If the bean property type is a primitive or a primitive wrapper, the the BeanFactory will convert the value to the appropriate primitive or primitive wrapper and then use that value when calling the setter. Some beans have properties with types that cannot automatically be converted from `String`. If the bean provides an alternative setter with the same name that does take a `String`, the BeanFactory will attempt to use that setter. If the BeanFactory cannot use the value or perform an appropriate conversion, setting the property will fail with a NamingException.

The `forceString` property available in earlier Tomcat releases has been removed as a security hardening measure.

#### Memory UserDatabase Resources

##### 0. Introduction

UserDatabase resources are typically configured as global resources for use by a UserDatabase realm. Tomcat includes a UserDatabaseFactory that creates UserDatabase resources backed by an XML file - usually `tomcat-users.xml`.

The steps required to set up a global UserDatabase resource are described below.

##### 1. Create/edit the XML file

The XML file is typically located at `$CATALINA_BASE/conf/tomcat-users.xml` however, you are free to locate the file anywhere on the file system. It is recommended that the XML files are placed in `$CATALINA_BASE/conf`. A typical XML would look like:

    <?xml version="1.0" encoding="UTF-8"?>
    <tomcat-users>
      <role rolename="tomcat"/>
      <role rolename="role1"/>
      <user username="tomcat" password="tomcat" roles="tomcat"/>
      <user username="both" password="tomcat" roles="tomcat,role1"/>
      <user username="role1" password="tomcat" roles="role1"/>
    </tomcat-users>

##### 2. Declare Your Resource

Next, modify `$CATALINA_BASE/conf/server.xml` to create the UserDatabase resource based on your XML file. It should look something like this:

    <Resource name="UserDatabase"
              auth="Container"
              type="org.apache.catalina.UserDatabase"
              description="User database that can be updated and saved"
              factory="org.apache.catalina.users.MemoryUserDatabaseFactory"
              pathname="conf/tomcat-users.xml"
              readonly="false" />

The `pathname` attribute can be a URL, an absolute path or a relative path. If relative, it is relative to `$CATALINA_BASE`.

The `readonly` attribute is optional and defaults to `true` if not supplied. If the XML is writable then it will be written to when Tomcat starts. WARNING: When the file is written it will inherit the default file permissions for the user Tomcat is running as. Ensure that these are appropriate to maintain the security of your installation.

If referenced in a Realm, the UserDatabase will, by default, monitor `pathname` for changes and reload the file if a change in the last modified time is observed. This can be disabled by setting the `watchSource` attribute to `false`.

##### 3. Configure the Realm

Configure a UserDatabase Realm to use this resource as described in the Realm configuration documentation.

#### DataSource UserDatabase Resources

##### 0. Introduction

Tomcat also include a `UserDatabase` that uses a `DataSource` resource as the backend. The backend resource must be declared in the same JNDI context as the user database that will use it.

The steps required to set up a global UserDatabase resource are described below.

##### 1. Database schema

The database shema for the user database is flexible. It can be the same as the schema used for the `DataSourceRealm`, with only a table for users (user name, password), and another one listing the roles associated with each user. To support the full `UserDatabase` features, it must include additional tables for groups, and is compatible with referential integrity between users, groups and roles.

The full featured schema with groups and referential integrity could be:

    create table users (
      user_name         varchar(32) not null primary key,
      user_pass         varchar(64) not null,
      user_fullname     varchar(128)
      -- Add more attributes as needed
    );

    create table roles (
      role_name         varchar(32) not null primary key,
      role_description  varchar(128)
    );

    create table groups (
      group_name        varchar(32) not null primary key,
      group_description varchar(128)
    );

    create table user_roles (
      user_name         varchar(32) references users(user_name),
      role_name         varchar(32) references roles(role_name),
      primary key (user_name, role_name)
    );

    create table user_groups (
      user_name         varchar(32) references users(user_name),
      group_name        varchar(32) references groups(group_name),
      primary key (user_name, group_name)
    );

    create table group_roles (
      group_name        varchar(32) references groups(group_name),
      role_name         varchar(32) references roles(role_name),
      primary key (group_name, role_name)
    );

The minimal schema without the ability to use groups will be (it is the same as for the `DataSourceRealm`):

    create table users (
      user_name         varchar(32) not null primary key,
      user_pass         varchar(64) not null,
      -- Add more attributes as needed
    );

    create table user_roles (
      user_name         varchar(32),
      role_name         varchar(32),
      primary key (user_name, role_name)
    );

##### 2. Declare Your Resource

Next, modify `$CATALINA_BASE/conf/server.xml` to create the UserDatabase resource based on your `DataSource` and its schema. It should look something like this:

    <Resource name="UserDatabase" auth="Container"
                  type="org.apache.catalina.UserDatabase"
                  description="User database that can be updated and saved"
                  factory="org.apache.catalina.users.DataSourceUserDatabaseFactory"
                  dataSourceName="jdbc/authority" readonly="false"
                  userTable="users" userNameCol="user_name" userCredCol="user_pass"
                  userRoleTable="user_roles" roleNameCol="role_name"
                  roleTable="roles" groupTable="groups" userGroupTable="user_groups"
                  groupRoleTable="group_roles" groupNameCol="group_name" />

The `dataSourceName` attribute is the JNDI name of the `DataSource` that will be the backend for the `UserDatabase`. It must be declared in the same JNDI `Context` as the `UserDatabase`. Please refer to the `DataSource` resources documentation for further instructions.

The `readonly` attribute is optional and defaults to `true` if not supplied. If the database is writable then changes made through the Tomcat management to the `UserDatabase` can be persisted to the database using the `save` operation.

Alternately, changes can also be made directly to the backend database.

##### 3. Resource configuration

Attribute | Description
--- | ---
`dataSourceName` | The name of the JNDI JDBC DataSource for this UserDatabase.
`groupNameCol` | Name of the column, in the "groups", "group roles" and "user groups" tables, that contains the group's name.
`groupRoleTable` | Name of the "group roles" table, which must contain columns named by the `groupNameCol` and `roleNameCol` attributes.
`groupTable` | Name of the "groups" table, which must contain columns named by the `groupNameCol` attribute.
`readonly` | If this is set to `true`, then changes to the `UserDatabase` can be persisted to the `DataSource` by using the `save` method. The default value is `true`.
`roleAndGroupDescriptionCol` | Name of the column, in the "roles" and "groups" tables, that contains the description for the roles and groups.
`roleNameCol` | Name of the column, in the "roles", "user roles" and "group roles" tables, which contains a role name assigned to the corresponding user. This attribute is required in majority of configurations. See allRolesMode attribute of the associated realm for a rare case when it can be omitted.
`roleTable` | Name of the "roles" table, which must contain columns named by the `roleNameCol` attribute.
`userCredCol` | Name of the column, in the "users" table, which contains the user's credentials (i.e. password). If a `CredentialHandler` is specified, this component will assume that the passwords have been encoded with the specified algorithm. Otherwise, they will be assumed to be in clear text.
`userGroupTable` | Name of the "user groups" table, which must contain columns named by the `userNameCol` and `groupNameCol` attributes.
`userNameCol` | Name of the column, in the "users", "user groups" and "user roles" tables, that contains the user's username.
`userFullNameCol` | Name of the column, in the "users" table, that contains the user's full name.
`userRoleTable` | Name of the "user roles" table, which must contain columns named by the `userNameCol` and `roleNameCol` attributes. This attribute is required in majority of configurations. See allRolesMode attribute of the associated realm for a rare case when it can be omitted.
`userTable` | Name of the "users" table, which must contain columns named by the `userNameCol` and `userCredCol` attributes.

##### 4. Configure the Realm

Configure a UserDatabase Realm to use this resource as described in the Realm configuration documentation.

#### Jakarta Mail Sessions

##### 0. Introduction

In many web applications, sending electronic mail messages is a required part of the system's functionality. The Jakarta Mail API makes this process relatively straightforward, but requires many configuration details that the client application must be aware of (including the name of the SMTP host to be used for message sending).

Tomcat includes a standard resource factory that will create `jakarta.mail.Session` session instances for you, already configured to connect to an SMTP server. In this way, the application is totally insulated from changes in the email server configuration environment - it simply asks for, and receives, a preconfigured session whenever needed.

The steps required for this are outlined below.

##### 1. Declare Your Resource Requirements

The first thing you should do is modify the web application deployment descriptor (`/WEB-INF/web.xml`) to declare the JNDI name under which you will look up preconfigured sessions. By convention, all such names should resolve to the `mail` subcontext (relative to the standard `java:comp/env` naming context that is the root of all provided resource factories. A typical `web.xml` entry might look like this:

    <resource-ref>
      <description>
        Resource reference to a factory for jakarta.mail.Session
        instances that may be used for sending electronic mail
        messages, preconfigured to connect to the appropriate
        SMTP server.
      </description>
      <res-ref-name>
        mail/Session
      </res-ref-name>
      <res-type>
        jakarta.mail.Session
      </res-type>
      <res-auth>
        Container
      </res-auth>
    </resource-ref>

WARNING - Be sure you respect the element ordering that is required by the DTD for web application deployment descriptors! See the Servlet Specification for details.

##### 2. Code Your Application's Use Of This Resource

A typical use of this resource reference might look like this:

    Context initCtx = new InitialContext();
    Context envCtx = (Context) initCtx.lookup("java:comp/env");
    Session session = (Session) envCtx.lookup("mail/Session");

    Message message = new MimeMessage(session);
    message.setFrom(new InternetAddress(request.getParameter("from")));
    InternetAddress to[] = new InternetAddress[1];
    to[0] = new InternetAddress(request.getParameter("to"));
    message.setRecipients(Message.RecipientType.TO, to);
    message.setSubject(request.getParameter("subject"));
    message.setContent(request.getParameter("content"), "text/plain");
    Transport.send(message);

Note that the application uses the same resource reference name that was declared in the web application deployment descriptor. This is matched up against the resource factory that is configured in the `<Context>` element for the web application as described below.

##### 3. Configure Tomcat's Resource Factory

To configure Tomcat's resource factory, add an elements like this to the `<Context>` element for this web application.

    <Context ...>
      ...
      <Resource name="mail/Session" auth="Container"
                type="jakarta.mail.Session"
                mail.smtp.host="localhost"/>
      ...
    </Context>

Note that the resource name (here, `mail/Session`) must match the value specified in the web application deployment descriptor. Customize the value of the `mail.smtp.host` parameter to point at the server that provides SMTP service for your network.

Additional resource attributes and values will be converted to properties and values and passed to `jakarta.mail.Session.getInstance(java.util.Properties)` as part of the `java.util.Properties` collection. In addition to the properties defined in Appendix A of the Jakarta Mail specification, individual providers may also support additional properties.

If the resource is configured with a `password` attribute and either a `mail.smtp.user` or `mail.user` attribute then Tomcat's resource factory will configure and add a `jakarta.mail.Authenticator` to the mail session.

##### 4. Install the Jakarta Mail API

Download the Jakarta Mail API.

Unpackage the distribution and place jakarta.mail-api-2.1.0.jar into $CATALINA_HOME/lib so that it is available to Tomcat during the initialization of the mail Session Resource. Note: placing this jar in both $CATALINA_HOME/lib and a web application's lib folder will cause an error, so ensure you have it in the $CATALINA_HOME/lib location only.

##### 5. Install a compatible implementaion

Select and download a compatible implementation.

Unpackage the implementation and place the jar file(s) into $CATALINA_HOME/lib.

Note: Other implementations may be available

##### 6. Restart Tomcat

For the additional JAR to be visible to Tomcat, it is necessary for the Tomcat instance to be restarted.

##### Example Application

The `/examples` application included with Tomcat contains an example of utilizing this resource factory. It is accessed via the "JSP Examples" link. The source code for the servlet that actually sends the mail message is in `/WEB-INF/classes/SendMailServlet.java`.

WARNING - The default configuration assumes that there is an SMTP server listing on port 25 on `localhost`. If this is not the case, edit the `<Context>` element for this web application and modify the parameter value for the `mail.smtp.host` parameter to be the host name of an SMTP server on your network.

#### JDBC Data Sources

##### 0. Introduction

Many web applications need to access a database via a JDBC driver, to support the functionality required by that application. The Jakarta EE Platform Specification requires Jakarta EE Application Servers to make available a DataSource implementation (that is, a connection pool for JDBC connections) for this purpose. Tomcat offers exactly the same support, so that database-based applications you develop on Tomcat using this service will run unchanged on any Jakarta EE server.

For information about JDBC, you should consult the following:

- http://www.oracle.com/technetwork/java/javase/jdbc/index.html - Home page for information about Java Database Connectivity.
- http://java.sun.com/j2se/1.3/docs/guide/jdbc/spec2/jdbc2.1.frame.html - The JDBC 2.1 API Specification.
- http://java.sun.com/products/jdbc/jdbc20.stdext.pdf - The JDBC 2.0 Standard Extension API (including the `javax.sql.DataSource` API). This package is now known as the "JDBC Optional Package".
- https://jakarta.ee/specifications/platform/9/ - The Jakarta EE Platform Specification (covers the JDBC facilities that all Jakarta EE platforms must provide to applications).

NOTE - The default data source support in Tomcat is based on the DBCP 2 connection pool from the Commons project. However, it is possible to use any other connection pool that implements `javax.sql.DataSource`, by writing your own custom resource factory, as described below.

##### 1. Install Your JDBC Driver

Use of the JDBC Data Sources JNDI Resource Factory requires that you make an appropriate JDBC driver available to both Tomcat internal classes and to your web application. This is most easily accomplished by installing the driver's JAR file(s) into the `$CATALINA_HOME/lib` directory, which makes the driver available both to the resource factory and to your application.

##### 2. Declare Your Resource Requirements

Next, modify the web application deployment descriptor (`/WEB-INF/web.xml`) to declare the JNDI name under which you will look up preconfigured data source. By convention, all such names should resolve to the `jdbc` subcontext (relative to the standard `java:comp/env` naming context that is the root of all provided resource factories. A typical `web.xml` entry might look like this:

    <resource-ref>
      <description>
        Resource reference to a factory for java.sql.Connection
        instances that may be used for talking to a particular
        database that is configured in the <Context>
        configuration for the web application.
      </description>
      <res-ref-name>
        jdbc/EmployeeDB
      </res-ref-name>
      <res-type>
        javax.sql.DataSource
      </res-type>
      <res-auth>
        Container
      </res-auth>
    </resource-ref>

WARNING - Be sure you respect the element ordering that is required by the DTD for web application deployment descriptors! See the Servlet Specification for details.

##### 3. Code Your Application's Use Of This Resource

A typical use of this resource reference might look like this:

    Context initCtx = new InitialContext();
    Context envCtx = (Context) initCtx.lookup("java:comp/env");
    DataSource ds = (DataSource)
      envCtx.lookup("jdbc/EmployeeDB");

    Connection conn = ds.getConnection();
    ... use this connection to access the database ...
    conn.close();

Note that the application uses the same resource reference name that was declared in the web application deployment descriptor. This is matched up against the resource factory that is configured in the `<Context>` element for the web application as described below.

##### 4. Configure Tomcat's Resource Factory

To configure Tomcat's resource factory, add an element like this to the `<Context>` element for the web application.

    <Context ...>
      ...
      <Resource name="jdbc/EmployeeDB"
                auth="Container"
                type="javax.sql.DataSource"
                username="dbusername"
                password="dbpassword"
                driverClassName="org.hsql.jdbcDriver"
                url="jdbc:HypersonicSQL:database"
                maxTotal="8"
                maxIdle="4"/>
      ...
    </Context>

Note that the resource name (here, `jdbc/EmployeeDB`) must match the value specified in the web application deployment descriptor.

This example assumes that you are using the HypersonicSQL database JDBC driver. Customize the `driverClassName` and `driverName` parameters to match your actual database's JDBC driver and connection URL.

The configuration properties for Tomcat's standard data source resource factory (`org.apache.tomcat.dbcp.dbcp2.BasicDataSourceFactory`) are as follows:

- driverClassName - Fully qualified Java class name of the JDBC driver to be used.
- username - Database username to be passed to our JDBC driver.
- password - Database password to be passed to our JDBC driver.
- url - Connection URL to be passed to our JDBC driver. (For backwards compatibility, the property `driverName` is also recognized.)
- initialSize - The initial number of connections that will be created in the pool during pool initialization. Default: 0
- maxTotal - The maximum number of connections that can be allocated from this pool at the same time. Default: 8
- minIdle - The minimum number of connections that will sit idle in this pool at the same time. Default: 0
- maxIdle - The maximum number of connections that can sit idle in this pool at the same time. Default: 8
- maxWaitMillis - 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: -1 (infinite)

Some additional properties handle connection validation:

- validationQuery - SQL query that can be used by the pool to validate connections before they are returned to the application. If specified, this query MUST be an SQL SELECT statement that returns at least one row.
- validationQueryTimeout - Timeout in seconds for the validation query to return. Default: -1 (infinite)
- testOnBorrow - true or false: whether a connection should be validated using the validation query each time it is borrowed from the pool. Default: true
- testOnReturn - true or false: whether a connection should be validated using the validation query each time it is returned to the pool. Default: false

The optional evictor thread is responsible for shrinking the pool by removing any connections which are idle for a long time. The evictor does not respect `minIdle`. Note that you do not need to activate the evictor thread if you only want the pool to shrink according to the configured `maxIdle` property.

The evictor is disabled by default and can be configured using the following properties:

- timeBetweenEvictionRunsMillis - The number of milliseconds between consecutive runs of the evictor. Default: -1 (disabled)
- numTestsPerEvictionRun - The number of connections that will be checked for idleness by the evictor during each run of the evictor. Default: 3
- minEvictableIdleTimeMillis - The idle time in milliseconds after which a connection can be removed from the pool by the evictor. Default: 30*60*1000 (30 minutes)
- testWhileIdle - true or false: whether a connection should be validated by the evictor thread using the validation query while sitting idle in the pool. Default: false

Another optional feature is the removal of abandoned connections. A connection is called abandoned if the application does not return it to the pool for a long time. The pool can close such connections automatically and remove them from the pool. This is a workaround for applications leaking connections.

The abandoning feature is disabled by default and can be configured using the following properties:

- removeAbandonedOnBorrow - true or false: whether to remove abandoned connections from the pool when a connection is borrowed. Default: false
- removeAbandonedOnMaintenance - true or false: whether to remove abandoned connections from the pool during pool maintenance. Default: false
- removeAbandonedTimeout - The number of seconds after which a borrowed connection is assumed to be abandoned. Default: 300
- logAbandoned - true or false: whether to log stack traces for application code which abandoned a statement or connection. This adds serious overhead. Default: false

Finally there are various properties that allow further fine tuning of the pool behaviour:

- defaultAutoCommit - true or false: default auto-commit state of the connections created by this pool. Default: true
- defaultReadOnly - true or false: default read-only state of the connections created by this pool. Default: false
- defaultTransactionIsolation - This sets the default transaction isolation level. Can be one of `NONE`, `READ_COMMITTED`, `READ_UNCOMMITTED`, `REPEATABLE_READ`, `SERIALIZABLE`. Default: no default set
- poolPreparedStatements - true or false: whether to pool PreparedStatements and CallableStatements. Default: false
- maxOpenPreparedStatements - The maximum number of open statements that can be allocated from the statement pool at the same time. Default: -1 (unlimited)
- defaultCatalog - The name of the default catalog. Default: not set
- connectionInitSqls - A list of SQL statements run once after a Connection is created. Separate multiple statements by semicolons (`;`). Default: no statement
- connectionProperties - A list of driver specific properties passed to the driver for creating connections. Each property is given as `name=value`, multiple properties are separated by semicolons (`;`). Default: no properties
- accessToUnderlyingConnectionAllowed - true or false: whether accessing the underlying connections is allowed. Default: false

For more details, please refer to the Commons DBCP 2 documentation.

### Adding Custom Resource Factories

If none of the standard resource factories meet your needs, you can write your own factory and integrate it into Tomcat, and then configure the use of this factory in the `<Context>` element for the web application. In the example below, we will create a factory that only knows how to create `com.mycompany.MyBean` beans from the Generic JavaBean Resources example above.

#### 1. Write A Resource Factory Class

You must write a class that implements the JNDI service provider `javax.naming.spi.ObjectFactory` interface. Every time your web application calls `lookup()` on a context entry that is bound to this factory (assuming that the factory is configured with `singleton="false"`), the `getObjectInstance()` method is called, with the following arguments:

- Object obj - The (possibly null) object containing location or reference information that can be used in creating an object. For Tomcat, this will always be an object of type `javax.naming.Reference`, which contains the class name of this factory class, as well as the configuration properties (from the `<Context>` for the web application) to use in creating objects to be returned.
- Name name - The name to which this factory is bound relative to `nameCtx`, or `null` if no name is specified.
- Context nameCtx - The context relative to which the `name` parameter is specified, or `null` if `name` is relative to the default initial context.
- Hashtable environment - The (possibly null) environment that is used in creating this object. This is generally ignored in Tomcat object factories.

To create a resource factory that knows how to produce `MyBean` instances, you might create a class like this:

    package com.mycompany;

    import java.util.Enumeration;
    import java.util.Hashtable;
    import javax.naming.Context;
    import javax.naming.Name;
    import javax.naming.NamingException;
    import javax.naming.RefAddr;
    import javax.naming.Reference;
    import javax.naming.spi.ObjectFactory;

    public class MyBeanFactory implements ObjectFactory {

      public Object getObjectInstance(Object obj,
          Name name2, Context nameCtx, Hashtable environment)
          throws NamingException {

          // Acquire an instance of our specified bean class
          MyBean bean = new MyBean();

          // Customize the bean properties from our attributes
          Reference ref = (Reference) obj;
          Enumeration addrs = ref.getAll();
          while (addrs.hasMoreElements()) {
              RefAddr addr = (RefAddr) addrs.nextElement();
              String name = addr.getType();
              String value = (String) addr.getContent();
              if (name.equals("foo")) {
                  bean.setFoo(value);
              } else if (name.equals("bar")) {
                  try {
                      bean.setBar(Integer.parseInt(value));
                  } catch (NumberFormatException e) {
                      throw new NamingException("Invalid 'bar' value " + value);
                  }
              }
          }

          // Return the customized instance
          return (bean);

      }

    }

In this example, we are unconditionally creating a new instance of the `com.mycompany.MyBean` class, and populating its properties based on the parameters included in the `<Resource>` element that configures this resource (see below). You should note that any parameter named `factory` should be skipped - that parameter is used to specify the name of the factory class itself (in this case, `com.mycompany.MyBeanFactory`) rather than a property of the bean being configured.

For more information about `ObjectFactory`, see the JNDI Service Provider Interface (SPI) Specification.

You will need to compile this class against a class path that includes all of the JAR files in the `$CATALINA_HOME/lib` directory. When you are through, place the factory class (and the corresponding bean class) unpacked under `$CATALINA_HOME/lib`, or in a JAR file inside `$CATALINA_HOME/lib`. In this way, the required class files are visible to both Catalina internal resources and your web application.

#### 2. Declare Your Resource Requirements

Next, modify your web application deployment descriptor (`/WEB-INF/web.xml`) to declare the JNDI name under which you will request new instances of this bean. The simplest approach is to use a `<resource-env-ref>` element, like this:

    <resource-env-ref>
      <description>
        Object factory for MyBean instances.
      </description>
      <resource-env-ref-name>
        bean/MyBeanFactory
      </resource-env-ref-name>
      <resource-env-ref-type>
        com.mycompany.MyBean
      </resource-env-ref-type>
    </resource-env-ref>

WARNING - Be sure you respect the element ordering that is required by the DTD for web application deployment descriptors! See the Servlet Specification for details.

#### 3. Code Your Application's Use Of This Resource

A typical use of this resource environment reference might look like this:

    Context initCtx = new InitialContext();
    Context envCtx = (Context) initCtx.lookup("java:comp/env");
    MyBean bean = (MyBean) envCtx.lookup("bean/MyBeanFactory");

    writer.println("foo = " + bean.getFoo() + ", bar = " +
                   bean.getBar());

#### 4. Configure Tomcat's Resource Factory

To configure Tomcat's resource factory, add an elements like this to the `<Context>` element for this web application.

    <Context ...>
      ...
      <Resource name="bean/MyBeanFactory" auth="Container"
                type="com.mycompany.MyBean"
                factory="com.mycompany.MyBeanFactory"
                singleton="false"
                bar="23"/>
      ...
    </Context>

Note that the resource name (here, `bean/MyBeanFactory` must match the value specified in the web application deployment descriptor. We are also initializing the value of the `bar` property, which will cause `setBar(23)` to be called before the new bean is returned. Because we are not initializing the `foo` property (although we could have), the bean will contain whatever default value is set up by its constructor.

You will also note that, from the application developer's perspective, the declaration of the resource environment reference, and the programming used to request new instances, is identical to the approach used for the Generic JavaBean Resources example. This illustrates one of the advantages of using JNDI resources to encapsulate functionality - you can change the underlying implementation without necessarily having to modify applications using the resources, as long as you maintain compatible APIs.