What is CGLIB?
A Byte Code Generation Library which is high level API to generate and transform Java byte code. It is used in various scenarios such as AOP, testing, data access frameworks to generate dynamic proxy objects and intercept field access.
See one example in unit test.
In line 17, a new dynamic proxy class is generated as mock.
In line 19, we tell the proxy, "if get(0) is called on this mock class, then return mocked data "hello, world".
As a result, in line 23 "result: hello, world" will be printed out.
In debugger you can find that the variable in line 17 is mocked by CGLIB:
Its byte code is generated dynamically and stored in variable byte[] b in line 217.
Let's see another example of injecting pre-exit and post-exit ( which ABAPers are very familiar with ) into a given method via dynamic proxy generated by CGLIB:
1. I have a class MyMEthodExitDemo which has a normal method myFun.
2. A new dynamic proxy class is generated in method createProxy which has a method with equal name as original class plus custom enhancement covered by class JerryEnhancement.
3. The pre-exit and post-exit are defined in class JerryEnhancement which implements interface MethodInterceptor defined in CGLIB library. The original method is generated in line 14, with pre-exit before it ( line 13 ) and post-exit after it ( line 15 ).
Execute result:
How does this example work under the hood?
I implement a demo in ABAP with the same logic.
1. I have a very simple ABAP class with only one public method which will print "Hello World":
2. The CGLIB utility class in line 5 simulates the logic in Java, which will construct a proxy class based on existing global class ZCL_JAVA_CGLIB. The generated proxy class will be DYNAMICALLY injected with two enhancement implemented by zcl_jerry_preexit and zcl_jerry_postexit.
There are two interfaces defined for pre-exit and post-exit logics which contain only one EXECUTE method without any parameters:
And zcl_jerry_preexit and zcl_jerry_postexit implement these two interfaces accordingly:
once method greet in line 16 is called, the enhanced version of method greet is called:
By this way, the method greet is enhanced in a non-invasive approach - no modification on original class, no new class are persisted in repository.
The method get_proxy of zcl_abap_cglib_tool does the magic, I just follow the idea of CGLIB implementation in Java:
The pre-exit and post-exit passed by consumer are dynamically injected into proxy class here:
Till now you should be familiar with CGLIB idea, and it is quite easy to understand why it is NOT possible to create a proxy class based on a final class by CGLIB, simply because a final class could not be subclassed.
The source code of zcl_abap_cglib_tool could be found from my github.
The report to consume it:
REPORT zcglib_proxy_generate.
DATA: lo_proxy TYPE REF TO object.
CALL METHOD zcl_abap_cglib_tool=>get_proxy
EXPORTING
iv_class_name = 'ZCL_JAVA_CGLIB'
io_pre_exit = NEW zcl_jerry_preexit( )
io_post_exit = NEW zcl_jerry_postexit( )
RECEIVING
ro_proxy = lo_proxy.
CHECK lo_proxy IS NOT INITIAL.
DATA(lo_class) = CAST zcl_java_cglib( lo_proxy ).
lo_class->greet( ).Update on 2017-02-02 CGLIB use scenario
The CGLIB is widely used in many Java framework, for example Mockito as a unit test framework. See my another blog Simulate Mockito in ABAP for detail.
Further reference
3. If for your use case you need a PERSISTENT proxy class instead, you can try approach introduced in this blog Create dynamic proxy persistently in Java and ABAP.
Further reading
I have written a series of blogs which compare the language feature among ABAP, JavaScript and Java. You can find a list of them below:
- Lazy Loading, Singleton and Bridge design pattern in JavaScript and in ABAP
- Fibonacci Sequence in ES5, ES6 and ABAP
- Java byte code and ABAP Load
- How to write a correct program rejected by compiler: Exception handling in Java and in ABAP
- An small example to learn Garbage collection in Java and in ABAP
- String Template in ABAP, ES6, Angular and React
- Try to access static private attribute via ABAP RTTI and Java Reflection
- Local class in ABAP, Java and JavaScript
- Integer in ABAP, Java and JavaScript
- Covariance in Java and simulation in ABAP
- Various Proxy Design Pattern implementation variants in Java and ABAP
- Implement CGLIB in ABAP
- SAP Managed Tags:
