How-To #4: Data access framework usage

Introduction

This article will provide you with details on how to use MRules built-in layer allowing reading and writing data from or to Java beans.

This layer can be used in a standalone way, it’s not coupled to the rule engine itself.

If you are familiar with anyone of the popular Java frameworks using reflection to access data layer, you won’t be surprised by the syntax, even if it’s slightly different sometimes because it offers more possibilities.

How it works

Concept

The property to access is represented by a String. For example:
myProperty.myNestedProperty“.

This String must be processed, meaning parsed and transformed to a specific Object. We call this step “compiling” the property. This is the role of the IPropertyCompiler utility.

When the String representation of the property is compiled, you obtain an instance of a class implementing the interface IProperty. This resulting instance is immutable and might be used several times, in a multi-threaded environment, to get or set the targeted property.

For example:

What is possible?

The property access framework shipped with MRules allows to perform all of the following operations:

Reading:

  • Read an unlimited number of nested properties in an Objects hierarchy.
  • Read Arrays elements, whatever is the number of dimensions of the array. The index of the element to read might be a constant or a value read from another property.
  • Lists are handled in the same way as Arrays.
  • Read Maps values, given the key. The key of the value to read might be a constant or a value read from another property.
  • Read an instance field without getter.
  • Invoke a method with zero or one argument. The argument might be a constant or a value read from another property.
  • Combine all the above, whatever is the protection of the element to access (even if private).
  • Extra: Arrays length can be accessed with the “length” keyword used as if it were a direct field access.

Writing:

  • Write any property of a bean in an Objects hierarchy, with no limitation of level.
  • Write an element of an Array. The Array will be automatically adjusted if too small.
  • Lists are handled in the same way as Arrays.
  • Set Maps values, given the key.
  • Write an instance field.

Indexed and mapped getters / setters:

Reading or Writing Arrays and Lists might be achieved in two ways:

  • A getter / setter which returns / sets directly the Array or List.
  • An indexed getter / setter, with an integer argument, returning or writing the corresponding element.

The same feature is available for Maps access, with an extra argument corresponding to the key.

This feature allows complying with Apache Common Beanutils possibilities, but we advise to limit its usage for when it’s absolutely necessary and to prefer the standard getters / setters.

Performances

Thanks to the two-phase life cycle of the IProperty beans and to a very optimized code, our framework offers impressive performances for data access: up to 40% faster than the Apache common bean utils.

Moreover, an extension allowing generating bytecode at runtime avoid the usage of reflection for public member accesses, and further improves the execution speed.

Limitations

As of MRules 1.9.3, following limitations apply:

  • Static fields cannot be directly accessed.
  • Static methods cannot be directly accessed.
  • Instance methods with more than one argument cannot be directly accessed.
  • Indexed and mapped getters / setters: only one dimension or level.

Usage

Basics

Reading an object hierarchy is as simple as separating all properties names to traverse with a dot.

For example: “myProperty.myNestedProperty.deeperProperty.alwaysDeeper.leafValue

Mapped properties

Accessing Maps values id done by using parenthesis. For example: “myMap('My Key')” or “myMap("My Key")“.

If the key value comes from another sub-property, just write “myMap(myBean.myKey)“.

Another way to access Mapped properties is to use the same syntax as basic access. For example : “myMap.myKey” will work fine. If the “myMap” property value is not an extension of Map with a “getMyKey()” method, then the framework will intent to use the string “myKey” as the key.

This second way has some disadvantages, so we advise to use the parenthesis syntax :

  • It’s clearer
  • Keys are not limited to Strings
  • Key values can be read from other properties.

Indexed properties

For Lists and Arrays, the syntax is the same. It’s also the same if the Java Bean model used indexes getters / setters or not.

Accessing an Array or List element is achieved by : “myArray['2']” or “myArray["2"]“.

Note: The syntax “myArray[2]” is accepted here, as an integer cannot be interpreted as a sub-property.

If the index value comes from another sub-property, just write “myArray[myBean.myIntegerProperty]“.

Direct access

MRules allows directly accessing fields or invoking methods. The syntax to achieve this operation uses the exclamation mark as a delimiter.

The field name or the method invocation instruction (name + parenthesis + argument) start and end with an exclamation mark.

For example:

  • Field: “!myField!“.
  • Method: “!myMethod()!” or “!myMethod('My Argument")!” or “!myMethod(myBean.myArgument)!“.

The last exclamation mark might be omitted when it’s obvious (the dot implies a closing exclamation mark), but take care that it does not change the meaning of the property !

For example:

  • Field: “!myField” is OK.
  • Method: “!myMethod()” is OK.
  • This one cannot be omitted : “!myMapField!("My Key")“.

Extra : the length of an array is accessed using the “!length!” field access: “myArray.!length“.

Mixing everything!

All possibilities above can be mixed, and allow to adapt to most of the Java Beans models.

Some detailed examples:

  • Accessing a bean, then Map, for which the key comes from the second element of an List: “myBean.myMap(myList[2])“.
  • Accessing a multi-dimensionnal array: “myArray[2][myBean.myIntegerProperty][!myMethod()]“.
  • Retrieving the size of a List: “myBean.myList.!size()“.
  • And even using it to access the element of another list: “myBean.myMainList[myBean.myList.!size()].myListElementProperty“.

Almost everything is possible, you are only limited by the complexity of your data model!

Apache Commons connector

You might work with an existing application, making intensive use of Java common bean utils. And you might want to improve speed and user experience.

We developed a connector just for you, allowing to replace part of the common bean utils code with ours. This allows making the legacy code based on bean utils using MRules data access framework, without refactoring.

Just add MRules framework in your classpath, and execute on unique instruction at startup:

org.apache.commons.beanutils.BeanUtilsBean.setInstance(new com.massa.util.commons.MBeanUtilsBean(true);

The boolean “true” indicates that the Apache bean utils syntax, which is slightly different of MRules one, should be used, still to avoid refactoring and unexpected results.

We are working on this connector, to replace more features of the legacy library. But we are progressing carefully, in order to not being introducing unexpected behaviors.

MRules specific syntax

MRules intensively uses the data access framework, as all inputs / outputs are based on Java Beans properties.

To handle features specific to the rule engine, an extended property compiler is used internally and enriches the syntax with the following:

Local variables

They are prefixed with the “$” character and can be read and written.

For example:

  • Accessing a variable: “$myVariable“.
  • Accessing an array with the index as a variable: “myArray[$myVariable]".
  • Accessing nested properties of a variable: “$myVariable.myNestedProperty“.

The “$” character must be the first of the property (or sub-property). The following is invalid: “myBean.$myVariable“.

Global variables

They are prefixed with the “#” character and are read only. By convention, global variables are upper case. The list of predefined global variables can be found here.

For example:

  • Accessing a variable: “#NOW”.
  • Accessing an map with the index as a variable: “myMap(#RULESETNAME)".
  • Accessing properties of a variable: “#RULESETNAME.!length()“.

The “#” character must be the first of the property  (or sub-property). The following is invalid: “myBean.#READBASE“.