The document provides an overview of Mule Expression Language (MEL), including its main uses and syntax. MEL allows accessing and manipulating message properties and context through expressions. Key points include: - MEL expressions can make decisions, select values, replace tokens, and act as scripts using message properties and context. - Syntax is Java-like with property expressions to access message fields. Operators include arithmetic, comparison, logical, and control flow statements. - Context objects model the message, server, Mule instance, and application to access environment properties in expressions. - Variables store expression values globally or across flows. Functions extract data through XPath and regex expressions.

1. Expression Language
in Mule

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.
 At the heart of MEL are property expressions of the form contextObject.property. These
provide easy access to properties of the Mule message and its environment. For example,
the expression “message.payload” represents the payload property of the message context
object. Its value is the message payload.
 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.
 The syntax of a MEL program largely follows Java, but MEL implements dynamic typing by
performing type coercion at runtime. Semicolons follow each statement except the last,
whether the statements are on one line or separate lines. Return statements are usually
unnecessary, because a MEL program returns the value of the last MEL statement executed.
 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 – Literals
MEL Literals – Boolean Literals
 Boolean literals are the values true and false. These are case sensitive.
MEL Literals – Null Literals
 A null literal takes the form null or nil. These are case sensitive.
MEL Literals – Type Literals
 Refer to any Java class by its fully qualified name or if it is one of the classes in the below classes, by its unqualified name.
References use the same dot notation as in Java, except that we must use $ rather than a dot to refer to a nested class.
 MEL automatically imports the following Java classes.
 java.lang.*
 java.io.*
 java.net.*
 java.util.*
 java.math.BigDecimal
 java.math.BigInteger
 javax.activation.DataHandler
 javax.activation.MimeType
 java.util.regex.Pattern
 org.mule.api.transformer.DataType
 org.mule.transformer.types.DataTypeFactory
MEL Key/Value Maps, Lists, and Arrays
 Maps are important in Mule Expression Language because much of the context you can work with comes in the form of
maps.

10. 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.
Accessing Map Data
 MEL provides a simpler way to access map items than java.util.Map provides. For example, Mule
associates a map containing properties set by the inbound endpoint processor with each message.
We can refer to this map as message.inboundProperties. To retrieve the inbound property with key
name foo, write 'message.inboundProperties[foo]'. If that property can be set (never the case with
inbound properties, but true of some properties in other maps), we can write
message.outboundProperties[foo]=message.payload on the left side of an assignment. If we try to
set a property that cannot be set,Mule indicates failure by throwing
org.mvel2.PropertyAccessException.

11. 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 and Functions
 Property expressions facilitate the use of properties of the Mule message and its environment as
operands in MEL expressions. They take the form contextObject.property. Context objects provide
logical groupings of the properties of the message and its environment.
 Functions provide ways to extract information that doesn’t already exist as a single value that can
be embodied in a property.
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]

12. Mule Expression Language – Context Object - Server
Server
 This object provides read-only access to the properties of the hardware, operating system, user, and
network interface listed in the table.
 For example, the value of 'server.userName' is a string representing the name of the user.
Name Description
fileSeparator
Character that separates components of a file path ( "/" on UNIX and ""
on Windows)
host Fully qualified domain name of the server
ip The IP address of the server
locale
Default locale (of type java.util.Locale) of the JRE (can access
server.locale.language and server.locale.country)
javaVersion JRE version
javaVendor JRE vendor name
osName Operating system name
osArch Operating system architecture
osVersion Operating system version
systemProperties Map of Java system properties
timeZone Default TimeZone (java.util.TimeZone) of the JRE
tmpDir Temporary directory for use by the JRE
userName User name
userHome User home directory
userDir
User working directory.For example, the value of 'server.userName' is a
string representing the name of the user.

13. Mule Expression Language – Context Object – Mule, App
Mule
 This object provides read-only access to the properties of the Mule instance listed in the
table.
 For example, the value of 'mule.version' is a string representing the Mule version.
App
 This object provides access to the properties of the Mule application listed in the table
 For example, the value of 'app.name' is a string representing the application name.
 For example, 'app.registry['foo']' refers to the object named foo in the Mule registry map.
You can set or retrieve its value.
Name Description
clusterId Cluster ID
home File system path to the home directory of the mule server installation
nodeId Cluster node ID
version Mule Version
Name Description
encoding Application default encoding (read-only)
name Application name (read-only)
standalone True if Mule is running standalone (read-only)
workdir Application work directory (read-only)
registry
Map representing the Mule registry (read/write).For example, the value of 'app.name' is a string
representing the application name.For example, 'app.registry['foo']' refers to the object named foo
the Mule registry map. You can set or retrieve its value.

14. 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)

15. 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']
 As a further shortcut, you can simply use foo as a variable name in a MEL expression. If there is no
local MEL variable called foo, the MEL processor looks for one in flowVars, then in sessionVars, before
failing. For example, if the MEL expression contains foo == 'cat' and there is no local MEL variable named
foo, but there is a foo key in flowVars, then the foo in the expression is equivalent to flowVars['foo'].
 Note, however, that we can turn this method of resolution off by including a configuration attribute in the
xml configuration file:
<configuration>
<expression-language autoResolveVariables="false">
</configuration>

16. 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.