Basic query for Java code

Basic query for Java code¶

Learn to write and run a simple CodeQL query using LGTM.

学习使用LGTM编写并运行一个简单的CodeQL查询。

About the query

The query we’re going to run performs a basic search of the code for if statements that are redundant, in the sense that they have an empty then branch. For example, code such as:

我们要运行的查询是对代码进行基本搜索，寻找多余的if语句，即它们有一个空的then分支。例如:

if (error) { }

Running the query

1. In the main search box on LGTM.com, search for the project you want to query. For tips, see Searching.

   在LGTM.com的主搜索框中，搜索你要查询的项目。有关提示，请参阅搜索。

2. Click the project in the search results.

   点击搜索结果中的项目。

3. Click Query this project.

   点击查询这个项目。

   This opens the query console. (For information about using this, see Using the query console.)

   这将打开查询控制台。有关使用该功能的信息，请参见使用查询控制台。

   Note

   Alternatively, you can go straight to the query console by clicking Query console (at the top of any page), selecting Java from the Language drop-down list, then choosing one or more projects to query from those displayed in the Project drop-down list.

   你可以直接进入查询控制台，点击查询控制台（在任何页面的顶部），从语言下拉列表中选择Java，然后从项目下拉列表中显示的项目中选择一个或多个项目进行查询。

4. Copy the following query into the text box in the query console:

   将以下查询复制到查询控制台的文本框中。

   import java
   
   from IfStmt ifstmt, Block block
   where ifstmt.getThen() = block and
     block.getNumStmt() = 0
   select ifstmt, "This 'if' statement is redundant."
   

   LGTM checks whether your query compiles and, if all is well, the Run button changes to green to indicate that you can go ahead and run the query.

   在查询框的下方列出了你要查询的项目名称，以及最近分析的项目提交的ID。右边是一个图标，表示查询操作的进度。

5. Click Run.

   The name of the project you are querying, and the ID of the most recently analyzed commit to the project, are listed below the query box. To the right of this is an icon that indicates the progress of the query operation:

   在查询框的下方列出了你要查询的项目名称，以及最近分析的项目提交的ID。右边是一个图标，表示查询操作的进度。

   Note

   Your query is always run against the most recently analyzed commit to the selected project.

   你的查询总是针对最近分析过的项目的提交运行。

   The query will take a few moments to return results. When the query completes, the results are displayed below the project name. The query results are listed in two columns, corresponding to the two expressions in the select clause of the query. The first column corresponds to the expression ifstmt and is linked to the location in the source code of the project where ifstmt occurs. The second column is the alert message.

   查询需要几分钟才能返回结果。当查询完成后，结果会显示在项目名称下面。查询结果列在两列中，对应于查询的选择子句中的两个表达式。第一列对应表达式ifstmt，并链接到ifstmt出现的项目源代码中的位置。第二列是警报信息。

   ➤ Example query results

   Note

   An ellipsis (…) at the bottom of the table indicates that the entire list is not displayed—click it to show more results.

   表格底部的省略号(…)表示没有显示整个列表-点击它以显示更多结果。

6. If any matching code is found, click a link in the ifstmt column to view the if statement in the code viewer.

   如果找到任何匹配的代码，请点击ifstmt列中的链接，在代码查看器中查看if语句。

   The matching if statement is highlighted with a yellow background in the code viewer. If any code in the file also matches a query from the standard query library for that language, you will see a red alert message at the appropriate point within the code.

   匹配的if语句会在代码查看器中以黄色背景高亮显示。如果文件中的任何代码也匹配了该语言的标准查询库中的查询，您将在代码中的适当位置看到一条红色的警报信息。

About the query structure

After the initial import statement, this simple query comprises three parts that serve similar purposes to the FROM, WHERE, and SELECT parts of an SQL query.

在初始导入语句之后，这个简单的查询由三个部分组成，其作用类似于SQL查询中的FROM、WHERE和SELECT部分。

Extend the query

Query writing is an inherently iterative process. You write a simple query and then, when you run it, you discover examples that you had not previously considered, or opportunities for improvement.

查询的编写本质上是一个迭代的过程。你写了一个简单的查询，然后，当你运行它时，你会发现你以前没有考虑过的例子，或者改进的机会。

Remove false positive results

Browsing the results of our basic query shows that it could be improved. Among the results you are likely to find examples of if statements with an else branch, where an empty then branch does serve a purpose. For example:

浏览我们的基本查询结果，发现它还可以改进。在这些结果中，你很可能会发现if语句带有else分支的例子，其中空的then分支确实有一定的作用。例如:

if (...) {
  ...
} else if ("-verbose".equals(option)) {
  // nothing to do - handled earlier
} else {
  error("unrecognized option");
}

In this case, identifying the if statement with the empty then branch as redundant is a false positive. One solution to this is to modify the query to ignore empty then branches if the if statement has an else branch.

在这种情况下，将带有空的then分支的if语句识别为多余的语句，是一种假阳性。一种解决方法是修改查询，如果if语句有 else分支，则忽略空then分支。

To exclude if statements that have an else branch:

要排除有else分支的if语句。

1. Extend the where clause to include the following extra condition:

   扩展where子句，加入以下额外条件:

   and not exists(ifstmt.getElse())
   

   The where clause is now:

   现在的where子句:

   where ifstmt.getThen() = block and
     block.getNumStmt() = 0 and
     not exists(ifstmt.getElse())
   

2. Click Run.

   There are now fewer results because if statements with an else branch are no longer included.

   现在结果减少了，因为不再包含带有 else 分支的 if 语句。

➤ See this in the query console

Further reading

• CodeQL queries for Java
• Example queries for Java
• CodeQL library reference for Java
• “QL language reference”
• “CodeQL tools”