JPA性能调优简介

2016 Nov 1

（主要参考Hibernate和EclipseLink的文档。）

Fetching strategies

对于entity的association到底应该lazy fetch还是eager fetch。需要为不同的场景指定合适的fetch策略。不该eager的时候eager fetch费内存；不该lazy的时候lazy fetch会增加和数据库间的通信次数。

The Hibernate recommendation is to statically mark all associations lazy and to use dynamic fetching strategies for eagerness. This is unfortunately at odds with the JPA specification which defines that all one-to-one and many-to-one associations should be eagerly fetched by default. Hibernate, as a JPA provider, honors that default.

Hibernate推荐把association的fetch策略先定义为lazy的（statically，比如@ManyToOne(fetch = FetchType.LAZY)），然后在需要eager fetch的时候再动态地指定fetch策略。

比如可以通过JPQL或者Criteria以Fetch Join的方式来eager fetch，

SELECT mag FROM Magazine mag LEFT JOIN FETCH mag.articles WHERE mag.id = 1
-- 如果不想返回重复结果，可以加上DISTINCT

builder.createQuery(Employee.class).from(Employee.class).fetch("projects", JoinType.LEFT)...

通过entity graph（@NamedEntityGraph）或者Hibernate profile（@FetchProfile）可以为不同的场景指定fetch的策略。

另外，在lazy fetch的情况下，还可以通过指定batch size来减少fetch的次数。

Pagination

如果结果集太大，则考虑通过setFirstResult()和setMaxResults()以“分页”的方式来返回结果集。

java jpa

工具推荐：TitleCap

2016 Oct 30

推荐一个可以自动将英文标题首字母大写的工具。支持多种首字母大写规则，包括Chicago Manual of Style，

Always capitalize the first and the last word.

Capitalize all nouns, pronouns, adjectives, verbs, adverbs, and subordinate conjunctions (“as”, “because”, “although”, “if”, etc.).

Lowercase all articles, coordinate conjunctions (“and”, “or”, “nor”), and prepositions regardless of length, when they are other than the first or last word.

Lowercase the “to” in an infinitive.

tool

Here Documents

2016 Oct 28

Chapter 19. Here Documents

A here document is a special-purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or a command, such as ftp, cat, or the ex text editor.

除了interactive program，一些non-interactive program也支持heredoc。vi也支持heredoc。

Choose a limit string sufficiently unusual that it will not occur anywhere in the command list and confuse matters.

The closing limit string, on the final line of a here document, must start in the first character position. There can be no leading whitespace. Trailing whitespace after the limit string likewise causes unexpected behavior. The whitespace prevents the limit string from being recognized.

The - option to mark a here document limit string («-LimitString) suppresses leading tabs (but not spaces) in the output. This may be useful in making a script more readable.

limit string的前后都不要加空格。否则会遇到类似unexpected end of file之类的错误。如果在脚本里用heredoc，为了缩进可以加上-选项（<<-LimitString），可以省略行开头的tab键。

For those tasks too complex for a here document, consider using the expect scripting language, which was specifically designed for feeding input into interactive programs.

太复杂的交互可以考虑用expect脚本来实现。

CLI

用sed直接修改原文件

2016 Oct 28

sed提供了-i选项可以“就地”（in-place）修改原始文件。

-i extension
Edit files in-place, saving backups with the specified extension. If a zero-length extension is given, no backup will be saved. It is not recommended
to give a zero-length extension when in-place editing files, as you risk corruption or partial content in situations where disk space is exhausted, etc.

sed -i .bak -e 's/foo/bar/' pom.xml # 备份原始文件
sed -i '' -e 's/foo/bar/' pom.xml   # 不做备份
# sed -i -e 's/foo/bar/' pom.xml    # OS X下有“bug”，会生成名为pom.xml-e的备份文件

CLI