Ref: https://blogs.sap.com/2016/11/25/get-to-know-camels-simple-expression-language-in-hci/
Introduction
Simple is a, well, simple expression language, that ships with the Apache Camel integration framework. The language is also available in SAP Cloud Integration (which I will refer to as CPI for the remainder of this post).
In CPI, Simple is mainly used to access the contents of the message being processed and its exchange, and to construct condition expressions in the Router step. The language does, however, have a few more tricks up its sleeve.
In this blog post, I will give you an overview of Simple and its features. I will also point out areas where CPI’s version of Simple differs from Apache Camel’s.
A note on Camel versions
Up until recently, CPI in the Cloud Foundry environment was based on a newer version of Apache Camel than CPI in the Neo environment (2.24 vs. 2.17). This meant that some Simple expressions would work in Cloud Foundry but fail in Neo. One example of this is date arithmetic through offsets (thank you to my fellow SAP Mentor Daniel Graversen for pointing this out to me).
CPI in Neo is currently being updated to the same version of Apache Camel as CPI in Cloud Foundry. Keep in mind, however, that the update is not simultaneous across all tenants, so your particular tenant might not have received it yet.
The basics
The most basic Simple expression is the ${ } placeholder containing a built-in variable. This example expression evaluates to the payload of the input message:
${body}
There is an alternative form, $simple{ }, which is partly supported in CPI. It does not work in Router conditions, however, so I will stick to the shorter ${ } form in this post.
In CPI, the ${ } placeholder can be inserted in e.g. the payload in a Content Modifier step or applied in the Query Editor, adding dynamic values to an OData resource path.
The ${ } placeholder can also be combined with Simple’s operators to produce boolean expressions, which you can then use as conditions in your Router steps.
Built-in variables
Simple’s built-in variables provide you with access to information such as the message payload and header fields. The list below is not exhaustive. For a list of all built-in variables, please refer to the Simple language documentation. However, keep in mind that some of them might not be supported in CPI.
For more information about the camelContext variable, please see this blog post.
According to the Simple language documentation, property.name is actually deprecated. However, the non-deprecated form, exchangeProperty.name, will cause an error in CPI if used in a Router condition. This might be fixed in a future update, but for now you should stick to the deprecated form.
It is also possible to access headers and properties as header[name], property[name] and exchangeProperty[name]. These forms do not work in Router conditions, though, so for consistency it makes sense to stick to the header.name and property.name forms.
Date and time
You can do date formatting in Simple using the date:command:pattern variable. The command part indicates where to get the date to format. To format the current date and time, use the now command. The pattern is a java.text.SimpleDateFormat date and time pattern string, which you might already be familiar with, if you are a Java developer. Here’s an example:
${date:now:dd-MM-yyyy HH:mm}
At the time of writing, this evaluates to 07-03-2021 12:15. The time is, however, 13:15. What gives? Let’s add the pattern letter z, in order to see the time zone:
${date:now:dd-MM-yyyy HH:mm z}
This evaluates to 07-03-2021 12:17 UTC. In other words, the time zone is Coordinated Universal Time (UTC).
To get the time in a specific time zone, use the date-with-timezone variable. Here’s an example using my time zone (CET):
${date-with-timezone:now:CET:dd-MM-yyyy HH:mm}
Right now, this evaluates to 07-03-2021 13:18, which is the correct time in Denmark.
To format dates stored in headers and properties, use the header.MyDateHeader and property.MyDateProperty commands, respectively.
Simple in CPI supports some date arithmetic operations through offsets. In this example, I get the formatted time in CET four and a half hours from now:
${date-with-timezone:now+4h30m:CET:dd-MM-yyyy HH:mm}
To learn more about the SimpleDateFormat pattern strings, please see the class’s API documentation.
Random numbers
Should you need a random number, Simple has you covered by way of the random variable. There are two ways to employ it:
random(max)random(min, max)
The first form generates a random integer between 0 (included) and max (excluded), while the second form generates a random integer between min (included) and max (excluded). To generate a random integer between 1 and 10, you would therefore use the following expression:
${random(1, 11)}
Operators
When adding non-XML condition routes to a Router step in your integration flow, you need to construct boolean expressions, i.e. expressions that evaluate to either true or false. The Simple language supports a range of operators, that you can use for this purpose. In the following, I will briefly describe each available operator. For a list of Simple operators supported in CPI at a given time, please refer to SAP’s documentation of the Router step. Please note that currently Simple in Apache Camel supports more operators than Simple in CPI.
When writing your expressions, please be aware that all literal values must be enclosed in single quotes, regardless of data type. Omitting the quotes will cause an error. If required, the data type of the right-hand value will be converted into the data type of the left-hand value. In other words, this is a valid, numeric comparison, even though it looks like a string comparison at first glance:
${property.MyNumericProperty} > '0'
Comparison operators
The Simple language offers you the familiar comparison operators: =, !=, >, >=, < and <=. For some reason, the equality operator in CPI’s version of Simple is a single equals sign, even though it’s a double equals sign in Camel’s Simple language documentation.
Please note that string comparison is case sensitive. There’s actually a case insensitive equals operator in the Simple language (=~), but at the moment, it is not supported in CPI. In order to compare strings without considering case, you can do this instead:
${property.MyStringProperty.toLowerCase()} = 'lower case value'
For more information about calling methods, see Calling methods below.
Logical operators
The two available logical operators are and and or. According to the Simple language documentation, the and and or forms are actually deprecated, but at the time of writing, the alternative forms (&& and ||) are not supported in CPI. Keep in mind, though, that this might very well change with future CPI updates.
You can combine multiple and and or operators in the same expression, but you cannot use parentheses for grouping. Consequently, if your boolean expression is long and complex, you are probably better off moving the logic to a Script step instead, for the sake of readability and maintainability.
contains/not contains
The contains operator tests whether a string contains another string, and not contains tests whether it doesn’t contain the other string. Here’s an example:
${property.MyStringProperty} contains 'test'
As was the case with string comparison, this operator is case sensitive. To disregard case, convert to upper or lower case by calling toUpperCase() or toLowerCase(), respectively.
regex/not regex
Regular expressions are supported in the Simple language via the operators regex and not regex. The former tests whether a value matches the provided regular expression, and the latter tests whether it doesn’t match. Here’s an example that tests whether a product code is formatted as five lower-case letters followed by three digits:
${property.ProductCode} regex '^[a-z]{5}\d{3}$'
in/not in
The in operator tests whether a given value occurs in a list of values, and the not in operator tests whether it doesn’t occur in the list. Here’s an example:
${property.MyNumericProperty} in '1,2,3,4,5'
Note how the entire list in enclosed in single quotes, and how the elements are separated by commas with no whitespace around them. The data type of the list elements will automatically be converted into the data type of the left-hand side value.
When evaluating strings, keep in mind that the comparison is case sensitive. To disregard case, convert the left-hand side to upper or lower case first:
${property.MyStringProperty.toLowerCase()} in 'abc,def'
Calling methods
As we saw in the above with the toLowerCase() method of Java class String, you can call methods on objects in your expressions using the familiar dot notation.
Simple provides this functionality by supporting a subset of another Apache language called OGNL. Here’s an example:
${property.MyStringProperty.substring(0,4)} = 'test'
Here, the substring method of class String is called, and two integer parameters are passed to the method.
For straightforward method calls with scalar parameters, this works well. However, if your method invocation is more complicated, e.g. if it requires creating and passing objects to the method, moving it to a Script step is the way to go.
Also, there is a special case you need to be aware of: Examples will show method calls without parentheses, and if you are calling a method that doesn’t take any parameters, you don’t need them, unless the method is overloaded. When calling an overloaded method that takes no parameters, you need to append a pair of empty parentheses. Otherwise the call will be ambiguous, and you will end up with a runtime error.
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