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:
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.
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 ${ }.
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 is supported:
| Operator |
Description |
| == |
equals |
| > |
greater than |
| >= |
greater than or equals |
| < |
less than |
| <= |
less than or equals |
| != |
not equals |
| contains |
For testing if contains in a string based value |
| not contains |
For testing if not contains in a string based value |
| regex |
For matching against a given regular _expression_ pattern defined as a String value |
| not regex |
For not matching against a given regular _expression_ pattern defined as a String value |
| in |
For matching if in a set of values, each element must be separated by comma. |
| not in |
For matching if not in a set of values, each element must be separated by comma. |
| is |
For matching if the left hand side type is an instanceof the value. |
| not is |
For matching if the left hand side type is not an instanceof the value. |
| range |
For matching if the left hand side is within a range of values defined as numbers: from..to |
| not range |
For matching if the left hand side is not within a range of values defined as numbers: from..to |
And the following operators can be used to group expressions:
| Operator |
Description |
| and |
and is used to group two expressions |
| or |
or is used to group two 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'")
simple("${in.header.foo} == foo")
simple("${in.header.bar} == '100'")
simple("${in.header.bar} == 100")
simple("${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 < ${in.header.bar}")
which then ensures the int type is used as 1st priority.
This may change in the future if the Camel team improves and let the binary comparision operations be smarter and prefer numeric types over String based. It's most often the String type which causes problem when comparing with numbers. |
simple("${in.header.baz} == 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("${in.header.type} not in 'gold,silver'")
And you can test for if the type is a certain instance, eg for instance a String
simple("${in.header.type} is 'java.lang.String'")
We have added a shorthand for all java.lang types so you can write it as:
simple("${in.header.type} is String")
Ranges is also supported. The range interval requires numbers and both from and end is inclusive. For instance to test whether a value is between 100 and 199:
simple("${in.header.number} range 100..199")
Notice we use .. in the range without spaces. Its based on the same syntax as Groovy.
![]() | 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 had 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>${in.header.type} == 'widget'</simple>
<to uri="bean:orderService?method=handleWidget"/>
</filter>
</from>
|
Using and / or
If you have two expressions you can combine them with the and or or operator.
For instance:
simple("${in.header.title} contains 'Camel' and ${in.header.type'} == 'gold'")
And of course the or is also supported. The sample example would be:
simple("${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 let Camel be able 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 is a few types which have a shorthand notation, hence why 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 exists null is returned.
Dependencies
The Simple language is part of camel-core.
