JDK 5 Documentation v1.2.2, Java 2 SDK 英文文档

JDBC^TM Guide: Getting Started

6 Database connections

For the full interface descriptions see the Java interfaces in Chapter 13.

6.1 Opening a connection

When you want to access a database, you may obtain a java.sql.Connection object from the JDBC management layer's java.sql.DriverManager.getConnection method.

The DriverManager.getConnection method takes a URL string as an argument. The JDBC management layer will attempt to locate a driver that can connect to the database represented by the URL. The JDBC management layer does this by asking each driver in turn (see Section 6.2 below) if it can connect to the given URL.¹ Drivers should examine the URL to see if it specifies a subprotocol that they support (see Section 6.3 below), and if so, they should attempt to connect to the specified database. If they succeed in establishing a connection, then they should return an appropriate java.sql.Connection object.

From the java.sql.Connection object it is possible to obtain java.sql.Statement, java.sql.PreparedStatement , and java.sql.CallableStatement objects that can be used to execute SQL statement s.

We also permit applications to bypass the JDBC management layer during connection open and explicitly select and use a particular driver.

6.2 Choosing between drivers

It may sometimes be the case that several JDBC drivers are capable of connecting to a given URL. For example, when connecting to a given remote database it might be possible to use either a JDBC-ODBC bridge driver, or a JDBC to generic network protocol driver, or to use a driver supplied by the database vendor.

JDBC allows users to specify a driver list by setting a Java property "jdbc.drivers". If this property is defined,then it should be a colon separated list of driver class names, such as "acme.wonder.Driver :foobaz.openNet.Driver:vendor.OurDriver".

When searching for a driver, JDBC will use the first driver it finds that can successfully connect to the given URL. It will first try to use each of the drivers specified in the sql.drivers list, in the order given. It will then proceed to try to use each loaded driver in the order in which the drivers were loaded. It will skip any drivers which are untrusted code, unless they have been loaded from the same source as the code that is trying to open the connection (see the security discussion in Section 5).

6.3 URLs

6.3.1 Goals for JDBC database naming

We need to provide a way of naming databases so that application writers can specify which database they wish to connect to.

We would like this JDBC naming mechanism to have the following properties:

Different drivers can use different schemes for naming databases. For example, a JDBC-ODBC bridge driver may support simple ODBC style data source names, whereas a network protocol driver may need to know additional information so it can discover which hostname and port to connect to.

If a user downloads an applet that wants to talk to a given database then we would like to be able to open a database connection without requiring the user to do any system administration chores. Thus for example, we want to avoid requiring an analogue of the human-administered ODBC data source tables on the client machines. This implies that it should be possible to encode any necessary connection information in the JDBC name.

We would like to allow a level of indirection in the JDBC name, so that the initial name may be resolved via some network naming system in order to locate the database. This will allow system administrators to avoid specifying particular hosts as part of the JDBC name. However, since there are a number of different network name services (such as NIS, DCE, etc.) we do not wish to mandate that any particular network nameserver is used.

6.3.2 URL syntax

Fortunately the World Wide Web has already standardized on a naming system that supports all of these properties. This is the Uniform Resource Locator (URL) mechanism. So we propose to use URLs for JDBC naming, and merely recommend some conventions for structuring JDBC URLs.

We recommend that JDBC URL's be structured as:

jdbc:<subprotocol>:<subname>

where a subprotocol names a particular kind of database connectivity mechanism that may be supported by one or more drivers. The contents and syntax of the subname will depend on the subprotocol.

If you are specifying a network address as part of your subname, we recommend following the standard URL naming convention of "//hostname:port/subsubname" for the subname. The subsubname can have arbitrary internal syntax.

6.3.3 Example URLs

For example, in order to access a database through a JDBC-ODBC bridge, one might use a URL like:

jdbc:odbc:fred

In this example the subprotocol is "odbc" and the subname is a local ODBC data source name "fred". A JDBC-ODBC driver can check for URLs that have subprotocol "odbc" and then use the subname in an ODBC SQLConnect.

If you are using some generic database connectivity protocol "dbnet" to talk to a database listener , you might have a URL like:

jdbc:dbnet://wombat:356/fred

In this example the URL specifies that we should use the "dbnet" protocol to connect to port 356 on host wombat and then present the subsubname "fred" to that port to locate the final database.

If you wish to use some network name service to provide a level of indirection in database names, then we recommend using the name of the naming service as the subprotocol. So for example one might have a URL like:

jdbc:dcenaming:accounts-payable

In this example, the URL specifies that we should use the local DCE naming service to resolve the database name "accounts-payable" into a more specific name that can be used to connect to the real database. In some situations, it might be appropriate to provide a pseudo driver that performed a name lookup via a network name server and then used the resulting information to locate the real driver and do the real connection open.

6.3.4 Drivers can choose a syntax and ignore other URLs.

In summary, the JDBC URL mechanism is intended to provide a framework so that different drivers can use different naming systems that are appropriate to their needs. Each driver need only understand a single URL naming syntax, and can happily reject any other URLs that it encounters.

6.3.5 Registering subprotocol names

JavaSoft will act as an informal registry for JDBC sub-protocol names. Send mail to jdbc@wombat.eng.sun.com to reserve a sub-protocol name.

6.3.6 The "odbc" subprotocol

The "odbc" subprotocol has been reserved for URLs that specify ODBC style Data Source Names. For this subprotocol we specify a URL syntax that allows arbitrary attribute values to be specified after the data source name.

The full odbc subprotocol URL syntax is:

jdbc:odbc:<data-source-name>[;<attribute-name>=<attribute-value>]*

Thus valid jdbc:odbc names include:

jdbc:odbc:qeor7

jdbc:odbc:wombat

jdbc:odbc:wombat;CacheSize=20;ExtensionCase=LOWER

jdbc:odbc:qeora;UID=kgh;PWD=fooey

6.4 Connection arguments

When opening a connection, you can pass in a java.util.Properties object. This object is a property set that maps between tag strings and value strings. Two conventional properties are "user" and "password". Particular drivers may specify and use other properties.

In order to allow applets to access databases in a generic way, we recommend that as much connection information as possible be encoded as part of the URL and that driver writers minimize their use of property sets.

6.5 Multiple connections

A single application can maintain multiple database connections to one or more databases, using one or more drivers.

6.6 Registering drivers

The JDBC management layer needs to know which database drivers are available. We provide two ways of doing this.

First, when the JDBC java.sql.DriverManager class initializes it will look for a "sql.drivers" property in the system properties. If the property exists it should consist of a colon-separated list of driver class names. Each of the named classes should implement the java.sql.Driver interface. The DriverManager class will attempt to load each named Driver class.

Second, a programmer can explicitly load a driver class using the standard Class.forName method. For example, to load the acme.db.Driver class you might do:

Class.forName("acme.db.Driver");

In both cases it is the responsibility of each newly loaded Driver class to register itself with the DriverManager, using the DriverManager.registerDriver method. This will allow the DriverManager to use the driver when it is attempting to make database connections.

For security reasons the JDBC management layer will keep track of which class loader provided which driver and when opening connections it will only use drivers that come from the local filesystem or from the same classloader as the code issuing the getConnection request.

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¹ At first glance this may seem inefficient, but keep in mind that this requires only a few procedure calls and string comparisons per connection since it is unlikely that dozens of drivers will concurrently be loaded.

jdbc@wombat.eng.sun.com or jdbc-odbc@wombat.eng.sun.com