TextPreprocessor transform

This topic tells you about the Application Accelerator TextPreprocessor transform in Tanzu Application Platform (commonly known as TAP).

The TextPreprocessor transform allows conditional inclusion or exclusion of lines of text in documents, based on SpEL expression conditions. It is inspired by the syntax of C-style preprocessor directives (#IF/#ENDIF).

Syntax reference

TextPreprocessor()

The TextPreprocessor doesn’t accept configuration parameter. It acts on (text) files that are input to it, typically through Include(), and uses the set of values visible at invocation time to evaluate SpEL expressions.

Behavior

The TextPreprocessor looks for lines that contain the following constructs:

• #IF(condition): evaluates the condition, using SpEL syntax, and discards all lines until a matching closing directive if it evaluates to false.

• #ELSE: if the previous conditions evaluated to false, the contents of this branch is retained.

• #ELIF(condition): if the previous conditions evaluated to false, it considers a new condition.

• #ENDIF: closes an #IF/#ELSE/#ELIF block. Every block requires a closing #ENDIF directive.

These constructs don’t have to be at start of the line. In particular, you can embed directives inside comments in the target language so that they don’t break parsing of files in raw form. All lines that contain directives are cut out from the resulting file.

The directives are case-sensitive, they must be in capitals. The #IF( characters must always appear as a block and have no space in between.

You can also nest directives. For example, the following two snippets are equivalent:

#IF(a && b)
something
#ENDIF
#IF(a && c)
something else
#ENDIF

#IF(a)
#IF(b)
something
#ENDIF
#IF(c)
something else
#ENDIF
#ENDIF

Example

This example includes pom.xml dependencies based on the value of some accelerator option, in this case, databaseType.

Example pom.xml:

  <!-- #IF(#databaseType == 'postgres') -->
  <dependency>
      <groupId>org.postgresql</groupId>
      <artifactId>postgresql</artifactId>
      <scope>runtime</scope>
  </dependency>
  <!-- #ENDIF -->
  <!-- #IF(#databaseType == 'mysql') -->
  <dependency>
      <groupId>com.mysql</groupId>
      <artifactId>mysql-connector-j</artifactId>
      <scope>runtime</scope>
  </dependency>
  <!-- #ENDIF -->

Example accelerator.axl:

engine {
  ...
  applyTo('pom.xml') {
    TextPreprocessor()
  }
  ...
}