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

JDBC^TM Guide: Getting Started

2 Goals and philosophy

This section outlines the main goals and philosophy driving the API design.

2.1 A SQL level API

JDBC is intended as a "call-level" SQL interface for Java. This means the focus is on executing raw SQL statements and retrieving their results. We expect that higher-level APIs will be defined as well, and these will probably be implemented on top of this base level. Examples of higher-level APIs are direct transparent mapping of tables to Java classes, semantic tree representations of more general queries, and an embedded SQL syntax for Java.

We expect that various application builder tools will emit code that uses our API. However we also intend that the API be usable by human programmers, especially because there is no other solution available for Java right now.

2.2 SQL Conformance

Database systems support a wide range of SQL syntax and semantics, and they are not consistent with each other on more advanced functionality such as outer joins and stored procedures. Hopefully with time the portion of SQL that is truly standard will expand to include more and more functionality. In the meantime, we take the following position:

JDBC allows any query string to be passed through to an underlying DBMS driver, so an application may use as much SQL functionality as desired at the risk of receiving an error on some DBMSs. In fact, an application query need not even be SQL, or it may be a specialized derivative of SQL, e.g. for document or image queries, designed for specific DBMSs.
In order to pass JDBC compliance tests and to be called "JDBC COMPLIANT(TM)" we require that a driver support at least ANSI SQL92 Entry Level. This gives applications that want wide portability a guaranteed least common denominator. We believe ANSI SQL-2 Entry Level is reasonably powerful and is reasonably widely supported today.

2.3 JDBC must be implementable on top of common database interfaces

We need to ensure that the JDBC SQL API can be implemented on top of common SQL level APIs, in particular ODBC. This requirement has colored some parts of the specification, notably the handling of OUT parameters and large blobs.

2.4 Provide a Java interface that is consistent with the rest of the Java system

There has been a very strong positive response to Java. To a large extent this seems to be because the language and the standard runtimes are perceived as being consistent, simple, and powerful.

As far as we can, we would like to provide a Java database interface that builds on and reinforces the style and virtues of the existing core Java classes.

2.5 Keep it simple

We would prefer to keep this base API as simple as possible, at least initially. In general we would prefer to provide a single mechanism for performing a particular task, and avoid providing duplicate mechanisms. We will extend the API later if any important functionality is missing.

2.6 Use strong, static typing wherever possible

We would prefer that the JDBC API be strongly typed, with as much type information as possible expressed statically. This allows for more error checking at compile time.

Because SQL is intrinsically dynamically typed, we may encounter type mismatch errors at run-time where for example a programmer expected a SELECT to return an integer result but the database returned a string "foo". However we would still prefer to allow programmers to express their type expectations at compile time, so that we can statically check as much as possible. We will also support dynamically typed interfaces where necessary (see particularly Chapter 14).

2.7 Keep the common cases simple

We would like to make sure that the common cases are simple, and that the uncommon cases are doable.

A common case is a programmer executing a simple SQL statement (such as a SELECT, INSERT, UPDATE or DELETE) without parameters, and then (in the case of SELECT statement) processing rows of simple result types. A SQL statement with IN parameters is also common.

Somewhat less common, but still important, is the case where a programmer invokes a SQL statement using INOUT or OUT parameters. We also need to support SQL statements that read or write multi-megabyte objects, and less common cases such as multiple result sets returned by a SQL statement.

We expect that metadata access (e.g. to discover result-set types, or to enumerate the procedures in a database) is comparatively rare and is mostly used by sophisticated programmers or by builder tools. Metadata functions are therefore documented at the end of the specification, along with dynamically-typed data access; the average programmer can skip these sections.

2.8 Use multiple methods to express multiple functionality

One style of interface design is to use a very small number of procedures and offer a large number of control flags as arguments to these procedures, so that they can be used to effect a wide range of different behavior.

In general the philosophy of the Java core classes has been to use different methods to express different functionality. This tends to lead to a larger number of methods, but makes each method easier to understand. This approach has the major advantage that programmers who are learning how to use the basic interface aren't confused by having to specify arguments related to more complex behaviors.

We've tried to adopt the same approach for the JDBC interface, and in general have preferred to use multiple methods rather than using multi-purpose methods with flag arguments.

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