【正文】 o anyone else. 2. If you take a piece of software under the GPL and use it as a part of your product, you can39。re not selling the site itself to a third party as a product. (Consulting panies fall into a weird quasispace, but no one has gone after them for using GPL software to date.) Why is it good to support this movement? There are two sides to this answer: one technical and one political. Technically, it39。s a free solution out there if they aren39。s proprietary solution. It39。t use GPL software for mercial purposes. Nothing is farther from the truth. What you can39。s still good enough for probably 90% of the emerce sites in the world, however. And version , which is in alpha as of this writing, addresses a number of these concerns, including rowlevel locking and transaction control. Putting Tomcat and MySQL Together Combining Tomcat and MySQL provides a powerful, reliable, and free platform that you can use to learn, develop, and deploy JSP applications. And, best of all, the code that you develop using this platform will run nicely using iPla and Oracle or WebSphere and SQL Server. As a learning tool the two together are almost reference implementations of their respective protocols (JSP and SQL). As a result, you won39。s say that the second part fails. You wouldn39。s, if you created an entry in the PURCHASE table with a user ID of 3, there would have to already be a user ID of 3 in the USER table or an error would occur. Similarly, you couldn39。s the catch? What are you not getting in MySQL that makes people turn to Oracle? Well, MySQL is a neat little package, but it is missing some things that would be nice to have in a perfect world. A major feature that MySQL does not offer is database consistency checking. You can use foreign key tags in your schema, but MySQL cheerfully ignores them. A lot of DB As I know would consider this a very bad thing. A foreign key constraint prevents you from creating inconsistent data. For example, let39。s a lot in an emerce transaction that you need to remember and track: ?A user39。 This kind of embedding gets very old very fast when you have to code a lot of pages. In addition, having to escape all of the quotation marks can lead to a lot of confusing and hardtofind errors if you leave out a backslash. Eventually, a stillbetter idea emerged. Suppose that you could bine the best of static HTML pages and with the interactive capabilities of servlets. The result was JavaServer Pages (on the Microsoft side, the result was Active Server Pages). As Figure shows, JSP is a plicated beast. In the next chapter, you39。t to say that JSP is a cureall and that using it will automatically make your Web site a paragon of engineering art. It39。 你可以在一個(gè)網(wǎng)站開發(fā) 免費(fèi) 地使用 GPL 代碼， 因?yàn)槟悴毁u自己的網(wǎng)站作 為一個(gè)產(chǎn)品給 第三方。 GPL 下一個(gè)軟件，并以此作為你的作品的 一部分，你不能 收取費(fèi)用超出復(fù)制的 產(chǎn)品。 從政治上來說 這是一個(gè)積極的事，因?yàn)樗３终紊系拇蠊?的 誠實(shí)。 此外，你可以享受你正在 支持開源軟件活動(dòng)的知識(shí)。 把 Tomcat 和 MySQL 的結(jié)合 Tomcat 和 MySQL 的結(jié)合為你用它來學(xué)習(xí)、開發(fā)和部署 JSP 應(yīng)用程序提供了一個(gè)強(qiáng)大的、可靠的和免費(fèi)的平臺(tái)。你不會(huì)要收取用戶，但沒有 ship 的項(xiàng)目。同樣， MySQL 的人提供他們自己的難題為什么 MySQL 是好的，但能夠回滾事務(wù)是（在我看來）重要以確保您有可用。 在諸如 Oracle 的數(shù)據(jù)庫里，如果你創(chuàng)建了一個(gè)在 PURCHASE 表 ID 為 3的用戶輸入數(shù)據(jù)庫，在 USER 表里可能 已經(jīng)有一個(gè) ID 為 3的用戶或錯(cuò)誤會(huì)發(fā)生。據(jù)我所知許多數(shù)據(jù)庫管理員會(huì)認(rèn)為這是一個(gè)很糟糕的事情。 MySQL 是可用于幾乎所有的電腦上有相當(dāng)好的能力是相當(dāng)輕量級(jí)的處理器，安裝方便 （ 10 分鐘，而不像 Oracle 需要多個(gè)小時(shí)）。 ?您希望能夠如用戶數(shù)據(jù)或產(chǎn)品快速訪問數(shù)據(jù)，即使有數(shù)千或上百萬數(shù)據(jù)。但開始要求他們記住他們五分鐘前做了什么，和他們顯露的比一個(gè)肥皂劇 里的人物失憶還快。 為什么我們需要的數(shù)據(jù)庫？ 好，一個(gè)原因就是為了讓拉里埃里森想到比爾蓋茨的時(shí)候，他的 Oracle 有能力保持自己百憂解。 正如你可以看到，巨大的能量是在現(xiàn)有的 JSP 世界里。 新的實(shí)例看看是否已經(jīng)有一個(gè)被稱為 user 的 對(duì)象實(shí)例在當(dāng)前連接的用戶會(huì)話對(duì)象的空間存