Simple Expression Language

The Simple Expression Language was a really simple language you can use, but has since grown more powerful. Its primarily intended for being a really small and simple language for evaluating Expression and Predicate without requiring any new dependencies or knowledge of XPath; so its ideal for testing in camel-core. Its ideal to cover 95% of the common use cases when you need a little bit of expression based script in your Camel routes.

However for much more complex use cases you are generally recommended to choose a more expressive and powerful language such as:

• JavaScript
• EL
• OGNL
• Mvel
• Groovy
• one of the supported Scripting Languages

The simple language uses ${body} placeholders for complex expressions where the expression contains constant literals. The ${ } placeholders can be omitted if the expression is only the token itself.

Alternative syntax From Camel 2.5 onwards you can also use the alternative syntax which uses $simple{ } as placeholders. This can be used in situations to avoid clashes when using for example Spring property placeholder together with Camel.

Configuring result type From Camel 2.8 onwards you can configure the result type of the Simple expression. For example to set the type as a java.lang.Boolean or a java.lang.Integer etc.

File language is now merged with Simple language From Camel 2.2 onwards, the File Language is now merged with Simple language which means you can use all the file syntax directly within the simple language.

Simple Language Changes in Camel 2.9 onwards The Simple language have been improved from Camel 2.9 onwards to use a better syntax parser, which can do index precise error messages, so you know exactly what is wrong and where the problem is. For example if you have made a typo in one of the operators, then previously the parser would not be able to detect this, and cause the evaluation to be true. There is a few changes in the syntax which are no longer backwards compatible. When using Simple language as a Predicate then the literal text must be enclosed in either single or double quotes. For example: "${body} == 'Camel'". Notice how we have single quotes around the literal. The old style of using "body" and "header.foo" to refer to the message body and header is @deprecated, and its encouraged to always use ${ } tokens for the built-in functions. The range operator now requires the range to be in single quote as well as shown: "${header.zip} between '30000..39999'".

To get the body of the in message: "body", or "in.body" or "${body}".

A complex expression must use ${ } placeholders, such as: "Hello ${in.header.name} how are you?".

You can have multiple functions in the same expression: "Hello ${in.header.name} this is ${in.header.me} speaking".
However you can not nest functions in Camel 2.8.x or older (i.e. having another ${ } placeholder in an existing, is not allowed).
From Camel 2.9 onwards you can nest functions.

Variables

OGNL expression support

Available as of Camel 2.3

The Simple and Bean language now supports a Camel OGNL notation for invoking beans in a chain like fashion.
Suppose the Message IN body contains a POJO which has a getAddress() method.

Then you can use Camel OGNL notation to access the address object:

simple("${body.address}")
simple("${body.address.street}")
simple("${body.address.zip}")

Camel understands the shorthand names for getters, but you can invoke any method or use the real name such as:

simple("${body.address}")
simple("${body.getAddress.getStreet}")
simple("${body.address.getZip}")
simple("${body.doSomething}")

You can also use the null safe operator (?.) to avoid NPE if for example the body does NOT have an address

simple("${body?.address?.street}")

Its also possible to index in Map or List types, so you can do:

simple("${body[foo].name}")

To assume the body is Map based and lookup the value with foo as key, and invoke the getName method on that value.

You can access the Map or List objects directly using their key name (with or without dots) :

simple("${body[foo]}")
simple("${body[this.is.foo]}")

Suppose there was no value with the key foo then you can use the null safe operator to avoid the NPE as shown:

simple("${body[foo]?.name}")

You can also access List types, for example to get lines from the address you can do:

simple("${body.address.lines[0]}")
simple("${body.address.lines[1]}")
simple("${body.address.lines[2]}")

There is a special last keyword which can be used to get the last value from a list.

simple("${body.address.lines[last]}")

And to get the 2nd last you can subtract a number, so we can use last-1 to indicate this:

simple("${body.address.lines[last-1]}")

And the 3rd last is of course:

simple("${body.address.lines[last-2]}")

And yes you can combine this with the operator support as shown below:

simple("${body.address.zip} > 1000")

Operator support

Available as of Camel 2.0
We added a basic set of operators supported in the simple language in Camel 2.0. The parser is limited to only support a single operator.

To enable it the left value must be enclosed in ${ }. The syntax is:

${leftValue} OP rightValue

Where the rightValue can be a String literal enclosed in ' ', null, a constant value or another expression enclosed in ${ }.

Important There must be spaces around the operator.

Camel will automatically type convert the rightValue type to the leftValue type, so its able to eg. convert a string into a numeric so you can use > comparison for numeric values.

The following operators are supported:

And the following unary operators can be used:

And the following logical operators can be used to group expressions:

Using and,or operators In Camel 2.4 or older the and or or can only be used once in a simple language expression. From Camel 2.5 onwards you can use these operators multiple times.

The syntax for AND is:

${leftValue} OP rightValue and ${leftValue} OP rightValue 

And the syntax for OR is:

${leftValue} OP rightValue or ${leftValue} OP rightValue 

Some examples:

simple("${in.header.foo} == 'foo'")

// here Camel will type convert '100' into the type of in.header.bar and if its an Integer '100' will also be converter to an Integer
simple("${in.header.bar} == '100'")

simple("${in.header.bar} == 100")

// 100 will be converter to the type of in.header.bar so we can do > comparison
simple("${in.header.bar} > 100")

Comparing with different types When you compare with different types such as String and int, then you have to take a bit care. Camel will use the type from the left hand side as 1st priority. And fallback to the right hand side type if both values couldn't be compared based on that type. This means you can flip the values to enforce a specific type. Suppose the bar value above is a String. Then you can flip the equation: simple("100 < ${in.header.bar}") which then ensures the int type is used as 1st priority. This may change in the future if the Camel team improves the binary comparison operations to prefer numeric types over String based. It's most often the String type which causes problem when comparing with numbers.

// testing for null
simple("${in.header.baz} == null")

// testing for not null
simple("${in.header.baz} != null")

And a bit more advanced example where the right value is another expression

simple("${in.header.date} == ${date:now:yyyyMMdd}")

simple("${in.header.type} == ${bean:orderService?method=getOrderType}")

And an example with contains, testing if the title contains the word Camel

simple("${in.header.title} contains 'Camel'")

And an example with regex, testing if the number header is a 4 digit value:

simple("${in.header.number} regex '\\d{4}'")

And finally an example if the header equals any of the values in the list. Each element must be separated by comma, and no space around.
This also works for numbers etc, as Camel will convert each element into the type of the left hand side.

simple("${in.header.type} in 'gold,silver'")

And for all the last 3 we also support the negate test using not:

simple("${in.header.type} not in 'gold,silver'")

And you can test if the type is a certain instance, eg for instance a String

simple("${in.header.type} is 'java.lang.String'")

We have added a shorthand for all java.lang types so you can write it as:

simple("${in.header.type} is 'String'")

Ranges are also supported. The range interval requires numbers and both from and end are inclusive. For instance to test whether a value is between 100 and 199:

simple("${in.header.number} range 100..199")

Notice we use .. in the range without spaces. Its based on the same syntax as Groovy.

From Camel 2.9 onwards the range value must be in single quotes

simple("${in.header.number} range '100..199'")

Can be used in Spring XML As the Spring XML does not have all the power as the Java DSL with all its various builder methods, you have to resort to use some other languages for testing with simple operators. Now you can do this with the simple language. In the sample below we want to test if the header is a widget order: <from uri="seda:orders"> <filter> <simple>${in.header.type} == 'widget'</simple> <to uri="bean:orderService?method=handleWidget"/> </filter> </from>

Using and / or

If you have two expressions you can combine them with the and or or operator.

Camel 2.9 onwards Use && or || from Camel 2.9 onwards.

For instance:

simple("${in.header.title} contains 'Camel' and ${in.header.type'} == 'gold'")

And of course the or is also supported. The sample would be:

simple("${in.header.title} contains 'Camel' or ${in.header.type'} == 'gold'")

Notice: Currently and or or can only be used once in a simple language expression. This might change in the future.
So you cannot do:

simple("${in.header.title} contains 'Camel' and ${in.header.type'} == 'gold' and ${in.header.number} range 100..200")

Samples

In the Spring XML sample below we filter based on a header value:

    <from uri="seda:orders">
       <filter>
           <simple>in.header.foo</simple>
           <to uri="mock:fooOrders"/>
       </filter>
    </from>

The Simple language can be used for the predicate test above in the Message Filter pattern, where we test if the in message has a foo header (a header with the key foo exists). If the expression evaluates to true then the message is routed to the mock:foo endpoint, otherwise its lost in the deep blue sea .

The same example in Java DSL:

    from("seda:orders")
        .filter().simple("${in.header.foo}").to("seda:fooOrders");

You can also use the simple language for simple text concatenations such as:

   from("direct:hello").transform().simple("Hello ${in.header.user} how are you?").to("mock:reply");

Notice that we must use ${ } placeholders in the expression now to allow Camel to parse it correctly.

And this sample uses the date command to output current date.

   from("direct:hello").transform().simple("The today is ${date:now:yyyyMMdd} and its a great day.").to("mock:reply");

And in the sample below we invoke the bean language to invoke a method on a bean to be included in the returned string:

   from("direct:order").transform().simple("OrderId: ${bean:orderIdGenerator}").to("mock:reply");

Where orderIdGenerator is the id of the bean registered in the Registry. If using Spring then its the Spring bean id.

If we want to declare which method to invoke on the order id generator bean we must prepend .method name such as below where we invoke the generateId method.

   from("direct:order").transform().simple("OrderId: ${bean:orderIdGenerator.generateId}").to("mock:reply");

And in Camel 2.0 we can use the ?method=methodname option that we are familiar with the Bean component itself:

   from("direct:order").transform().simple("OrderId: ${bean:orderIdGenerator?method=generateId}").to("mock:reply");

And from Camel 2.3 onwards you can also convert the body to a given type, for example to ensure its a String you can do:

  <transform>
    <simple>Hello ${bodyAs(String)} how are you?</simple>
  </transform>

There are a few types which have a shorthand notation, so we can use String instead of java.lang.String. These are: byte[], String, Integer, Long. All other types must use their FQN name, e.g. org.w3c.dom.Document.

Its also possible to lookup a value from a header Map in Camel 2.3 onwards:

  <transform>
    <simple>The gold value is ${header.type[gold]}</simple>
  </transform>

In the code above we lookup the header with name type and regard it as a java.util.Map and we then lookup with the key gold and return the value.
If the header is not convertible to Map an exception is thrown. If the header with name type does not exist null is returned.

From Camel 2.9 onwards you can nest functions, such as shown below:

<setHeader headerName="myHeader">
  <simple>${properties:${header.someKey}}</simple>
</setHeader>

Setting result type

Available as of Camel 2.8

You can now provide a result type to the Simple expression, which means the result of the evaluation will be converted to the desired type. This is most useable to define types such as booleans, integers, etc.

For example to set a header as a boolean type you can do:

.setHeader("cool", simple("true", Boolean.class))

And in XML DSL

      <setHeader headerName="cool">
        <!-- use resultType to indicate that the type should be a java.lang.Boolean -->
        <simple resultType="java.lang.Boolean">true</simple>
      </setHeader>

Changing function start and end tokens

Available as of Camel 2.9.1

You can configure the function start and end tokens - ${ } using the setters changeFunctionStartToken and changeFunctionEndToken on SimpleLanguage, using Java code. From Spring XML you can define a <bean> tag with the new changed tokens in the properties as shown below:

    <!-- configure Simple to use custom prefix/suffix tokens -->
    <bean id="simple" class="org.apache.camel.language.simple.SimpleLanguage">
      <property name="functionStartToken" value="["/>
      <property name="functionEndToken" value="]"/>
    </bean>

In the example above we use [ ] as the changed tokens.

Notice by changing the start/end token you change those in all the Camel applications which share the same camel-core on their classpath.
For example in an OSGi server this may affect many applications, where as a Web Application as a WAR file it only affects the Web Application.

Dependencies

The Simple language is part of camel-core.