a simple Java-Expression-Language parser / EL / Java EL

Summary

-1-
provide you the possibility to configure your programs with Dynamic parameters.
for example, a configure-file looks like :

some.a=1
some.property=${Math.max(256,Runtime.getRuntime().availableProcessors()*8)}
some.b=2

the properties can be distributed by a configure-server or a disk file, and can be used as :

tomcat.setMaxThreads(EL.eval(config.get("some.property")));

will make your tomcat service run with correct threads number based on the machine it is running in.
this makes you use proper CPU/Memory resources for you application when it is running in different server, not a static value for all different servers in a cluster.

-2-
a command line program EL.main() can be used to parse user input, and test the logical before you write them into your code.

for example:

java -cp az-el.jar az.el.EL -e a=1 System.out.println(a)

will evaluate those 2 expressions. and:

java -cp az-el.jar az.el.EL -f /home/user/file.txt

will evaluate expressions in the file, line by line. and:

java -cp az-el.jar az.el.EL -i

will evaluate expressions you entered on console. this can be used as a java-shell.

Data Types

data types supported:

type	examples
bool/boolean	true, false
int/integer	1, 100 etc.
long	2L, 100L etc.
float	2.0f etc.
string	'abc' etc.

to supply a string in expression, please use a pair of single quotation marks,
for example:

EL.eval(new String[]{"System.out.print('some string here\n');"})

Special Variables

variable	description
?	the result of previous expression, any type
$	the current element when iterating through a collection
context	the execution context, which contains all variables

other variables which defined by user can be referenced by its name,
for example:

EL.eval("a=1;System.out.println(a*2)") 

will print out "2" on console.

strings should be enclosed with a pair of '', for example:

EL.eval("a='xyz';print(a)")

will print out a string "xyz" on console.
and do not name your variables with a start character "$".

Embedded Functions

1: map()
helps creating a Map object, for example:

EL.eval("result=map('a',1,'b','2')") 

will create a Map<Object,Object> named "result" with two entries of {"a"->1,"b"->"2"}.

2: list()
to create a List object, for example:

EL.eval("list(1,'a',2,3)")

will create a List<Object> collection object with 4 items in it.

3: array()
create an array, for example : array(1,2,'3').

4: each(), every(), foreach()
can be used to iterate over a collection, like list/array. for example:

EL.eval("each(a_list,'System.out.println($)')")

will print out each element on console. the collection can be an object of type Iterator/Iterable/List/Set/Map. if you just want to print out each element of a collection, use "each(a_list_object)", the same as "each(a_list_object,'println($)')".

5: iif()
used to check given condition and then return corresponding value. for example:

x=iif(a>1, obj.doSomething(a), obj.doAnother(a))

will evaluate the sub-expression "obj.doSomething(a)" only if "a>1", and never evaluate "obj.doAnother(a)" anyway. we delayed the expression-evaluation.

6: print(), println(), printf()
used to print something out on console, similar method-signature as System.out.print*() series.

Supported operators

+ - * / % & | ! ^ ~ >> << =
!= == > < >= <= ++ --
a+=1 a-=1 a*=1 a/=1 a%=2 a&=1 a|=1 a^=1

Special characters

char	description
;	semicolon
\n	line-feed

you can't feed expressions to EL.eval(String exp) method, if the string contains (;\n), use EL.eval(String[] exp) instead.

Method invoking

for example :

// for : Math.max(int a, int b).
Math.max#int(a,b) 
Math.max(long_var_1,long_var_2)
// for : Some.add(int a, int b).
var.add#int#int(1,2) 

you can specify a method what is matching the parameters-list you given, to overcome the method-overloading problem.

for example:

// for : print out a integer value.
System.out.println#int(...) 
// for : print out a float value.
System.out.println#float(...) 

Black-list / White-list classes

it's possible to provide a list of classes which shouldn't be used in code.

context.put(EL.VAR_NAME_CLASS_BLACKLIST, "a.SomeClass,b.*")

to disable some classes.

and you can setup a white-list

context.put(EL.VAR_NAME_CLASS_WHITELIST, "a.SomeClass,b.*")

to narrow down what class can be used in expression.

Expression Examples

"java.lang.System.out.print#string('hello');System.out.println('world')"
"java.lang.Math.max#int(1,2)"
"com.some.Class.prop='word'"
"com.some.Class.method#string#int('a',1,?)"
"var=com.SomeClass.new#string('jacky');var.sayHello('iAmLucy')"
"2*Math.max#int(5*Runtime.getRuntime().availableProcessors(),8*3)+3*4-1"
"iif(a>b,c,com.SomeClass.doSomething('x'));"
"var=array('a','b','c');each(var,'System.out.println#string($)');"
"foreach(map('a',1,'b',2))"

Issues ?

do not use very long expressions, it's much better to be shorter than 128 chars.

contact me through : arrenzhang@hotmail.com .