Mule Expression Language (MEL) is a separate expression Language used by Mule ESB to alter the Business data or read the metadata of Business data. Using MEL we can even alter the data.

1. Mule Expression Language

2. Mule Expression Language(MEL) - Introduction
 Mule Expression Language (MEL) supports the work of message processors by
providing a means of accessing, manipulating, and using information from the
message and its environment.
 In most cases, Mule expressions work within message processors to modify the way
those processors do their main jobs (for example, routing, filtering). Following are
the principal use cases:
 Make a decision based on the contents, properties, or context of a message and its
attachments. For example, a flow controller can route purchase orders for different types
of products to different JMS queues.
 Select a value from the contents, properties, or context of a message and its attachments.
For example, a cloud connector might extract a specific piece of information from the
current message to use as an argument.
 Replace a token with an actual value. For example, a logger can extract information from
the payload contents and place it into a logging message.
 We can also use MEL to implement a program (script) for which we would
otherwise use a programming language like Ruby, JavaScript, or Groovy. Scripts in
those languages cannot access the message and its environment as conveniently as
MEL can. In this case, MEL is the programming language for a custom message
processor.

3. Mule Expression Language - Syntax
Mule Expression Language Syntax
 MEL expression combines one or more operands with zero or more operators in a Java-like
syntax and returns the resulting value.
 Java method invocations and assignments are the other common MEL expressions.
 In most cases, a MEL expression stands alone as the value of a configuration property of a
message processor. Mule evaluates the expression at runtime, and the message processor
uses the result.
 MEL operators and basic operands are conventional and predictable (for example, 2 + 2 == 4
returns true). Property expressions provide convenient access to information from the
message and its environment (for example, server.fileSeparator returns "/" if the application is
running on a Linux server, and "" on a Windows server.). The remainder of this section
summarizes the most important elements of MEL syntax.

4. Mule Expression Language – Operand, Property
MEL Operand
 An operand can be a literal, a variable, or a MEL expression.
 The MEL expressions that most commonly appear as operands are property expressions and
method invocations.
Property Expressions
 The syntax of a property expression is “contextObject.property”. This can appear as an
operand in most MEL expressions, including on the left side of an assignment if the property is
writable from MEL.
Method Invocations
 Mule Expression Language uses standard Java method invocation.
 We can provide a fully qualified class name or import a class and use the unqualified name.
MEL automatically imports a number of Java classes.
 Eg. message.payload.getName(). If payload is a Java object—for example, representing a
person—this code invokes its getName method. The value of the expression is the value that
getName returns—presumably a string representing the person’s name.
MEL Operators
 MEL operators follow standard Java syntax, but operands are always by value, not by
reference. For example, "A" == 'A' evaluates to true, whereas the same expression evaluates
to false in Java.

5. Arithmetic Operators
Mule Expression Language – Arithmetic Operators
Symbol Definition Example/Value
-
Minus. The value is the value of the first operand minus the
value of the second.
2 - 4
-2
%
Modulo. The value is the remainder after dividing the value
of the first operand by the value of the second.
9 % 4
1
/
Over. The value is the value of the first operand divided by
the value of the second.
2 / 4
0.5
+
Plus. For numbers, the value is the sum of the values of the
operands. For strings, the value is the string formed by
concatenating the values of the operands.
2 + 4
6
'fu' + 'bar'
The String "fubar"
*
Times. The value is the product of the values of the
operands.
2 * 4
8

6. Comparison Operators
Mule Expression Language – Comparison Operators
Symbol Definition Example/Value
==
Equal. True if and only if (iff) the values of the operands are
equal.
'A' == 'A'
true
!= Not equal. True iff the values of the operands are unequal.
'A' != 'B'
true
>
Greater than. True iff the value on the left is greater than the
value on the right.
7 > 5
true
<
Less than. True iff the value on the left is less than the value
on the right
5 < 5
false
>=
Greater than or equal. True iff the value on the left is greater
than or equal to the value on the right.
5 >= 7
false
<=
Less than or equal. True iff the value on the left is less than or
equal to the value on the right.
5 <= 5
true
contains
Contains. True iff the string on the right is a substring of the
string on the left.
'fubar' contains
'bar'
true
is,
instance
of
Is an instance of. True iff the object on the left is an instance
of the class on the right.
'fubar' is String
true

7. Comparison Operators - contd
Logical Operators
Mule Expression Language – Logical Operators
Symbol Definition Example/Value
strsim
Degree of similarity. The value of the expression is a
number between 0 and 1 representing the degree of
similarity between the two string arguments.
'foo' strsim 'foo'
1.0
‘foobar’ strsim ‘foo’
0.5
soundslike
Sounds like. True iff the two string arguments sound alike
according to a Soundex comparison.
'Robert' soundslike
'Rupert'
true
Symbol Definition Example/Value
and
Logical AND. True iff both operands are true. (Don’t use
&&)
(a == b) and (c != d)
true iff a =b and c ≠ d
|| Logical OR. True iff at least one operand is true.
true ||anything
Always true
or
Chained OR. Scans left to right and returns the value of
the first non-empty item
false or '' or ' ' or 'dog'
The String "dog"

8. Mule Expression Language – Assignment, Literals
MEL Assignment
 An assignment is a MEL expression consisting of an identifier representing a mutable object to
the left of an equal sign and a MEL expression to the right of the equal sign. Eg:
message.payload = 'fu‘ sets the payload of the current message to the string "fu".
 MEL determines types dynamically, so declaring the type of a variable is optional. For example
if, with no prior declarations, if we write number = 1; number == '1' MEL assigns the
expression the value true.
 We can cast values to specific types. For example if we write
number = (String)1; number is String
MEL returns the value true for this expression.
MEL Literals
 Literals in MEL can be strings, numbers, Boolean values, types, and nulls. Maps, Lists and
Arrays data structures as literals as well.
MEL Literals – Numeric Literals
 Numeric literals are integers and floating point numbers, with the same ranges of values as
the underlying Java system.
MEL Literals – String Literals
 String literals are sequences of characters enclosed in single quotes. We cannot use double
quotes to express String literals as we can in Java, because MEL expressions appear within
double quotes in configuration files.

9. Mule Expression Language – Maps, Lists and Arrays
MEL Key/Value Maps, Lists, and Arrays
 Mule Expression Language uses a convenient syntax for maps and other data structures. It
begins with map literals, and there is also a convenient way to access items in maps. MEL
provides a streamlined way to access map data. Rather than constructing a map with a new
statement, and then using its put method to populate it, we can simply write the following:
[key1 : value1, key2 : value2, . . .]
and use this literal form wherever we would otherwise use a map by name, including as a
method argument.
 similar literal forms for lists ([item1, item2, . . .]) and arrays ({item1, item2, . . .}).
 Arrays in Java must specify the type of their contents, but in MEL they are untyped. MEL
supplies the correct type when we use them – either by determining it at compile time or
coercing the array to the correct type at run time.

10. Mule Expression Language–Control Flow, Context Objects
Control Flow
 MEL provides a full range of Java control flow statements. The most useful for typical MEL
expressions are conditional operands (often called ternary statements).
 A conditional operand has the form condition ? true value : false value.
For example, x = (name == 'Smith' ? 'Smith' : 'Unknown') sets the variable x to the string
"Smith" if the value of name is "Smith" and to the string "Unknown" if the value of name
is not "Smith".
MEL Context Objects
 Context objects model the message and its environment. They make MEL Mule-centric, not
just another expression language. Different context objects are
 Server: properties of the hardware, operating system, user, and network interface.
 Mule: properties of the Mule instance.
 App: properties of the Mule application.
 Message: properties of the Mule message - [DEFAULT]

11. Mule Expression Language – Context Object – Message
Message
 This object provides access to the properties of the Mule message listed in the table.
Name Description
id (read-only)
rootId (read-only)
correlationId (read-only)
correlationSequence (read-only)
correlationGroupSize (read-only)
replyTo (read/write)
dataType (read-only)
payload (read/write)
inboundProperties Map (read-only)
inboundAttachments Map (read-only)
outboundProperties Map (read/write)
outboundAttachments Map (read/write)
exception (read-only)

12. Mule Expression Language – Variables
Variables
In addition to local MEL variables, whose scope is the current message
processor, MEL gives you access to Mule flow and session variables. The
variables reside in the following maps, which are available to use in MEL
expressions:
 flowVars – contains variables that are global to the current flow. They retain their values
as control passes from one message processor to another. Thus, you can set them in one
message processor and use them in another.
 SessionVars – is essentially the same as flowVars, except that when one flow calls another
one via a Mule endpoint they are propagated.
For example, to access the value of the foo flow variable, write
flowVars['foo']. This can appear on either side of an assignment. For
example, the following code gets the value of the session variable bar and
uses it to set the value of the flow variable bar.
flowVars['foo'] = sessionVars['bar']

13. Mule Expression Language – Data Extraction Function
Data Extraction Function
 The functions xpath and regex provide ways of extracting context information extract
information that doesn’t already exist as a single value that can be embodied in a property. By
default they work on the payload, but we can pass them different arguments explicitly.

14. Thank you