Primitives and its wrapped types compatibility

IntroductionHow often do you think about possible changes in your API? Do you consider that something required could become optional in future? How about compatibility of such change? One of this changes is going from primitive (e. g. int) to its wrapp…

Introduction

How often do you think about possible changes in your API? Do you consider that something required could become optional in future? How about compatibility of such change? One of this changes is going from primitive (e. g. int) to its wrapped type (e. g. Integer). Let’s check it out.

API – first iteration

Let’s start with simple DTO class Dep in our public API.

public class Dep {
    private int f1;

public int getF1(){
return f1;
}

public void setF1(int f1){
this.f1 = f1;
}

// other fields and methods omitted
}

f1 is obligatory field that never will be null.

Let’s use it in Main class:

public class Main {
    public static void main(String... args) {
        Dep dep = new Dep();
        dep.setF1(123);
        System.out.println(dep.getF1());
    }
}

compile it:

$ javac depInt/Dep.java
$ javac -cp depInt main/Main.java

and run:

$ java -cp depInt:main Main
123

It works.

API – obligatory field become optional

Now suppose our business requirements have changed. f1 is not longer obligatory and we want possibility to set it to null.

So we provide next iteration of Dep class where f1 field has type Integer.

public class Dep {
    private Integer f1;

public Integer getF1(){
return f1;
}

public void setF1(Integer f1){
this.f1 = f1;
}

// other fields and methods omitted
}

We compile only the new Dep class because we do not want to change the Main class:

$ javac depInteger/Dep.java

and run it with old Main:

$ java -cp depInteger:main Main
Exception in thread "main" java.lang.NoSuchMethodError: Dep.setF1(I)V
    at Main.main(Main.java:4)

Wow! It does not work…

Why does it not work?

We can use javap tool to investigate Main class.

$ javap -c main/Main.class
Compiled from "Main.java"
public class Main {
  public Main();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

public static void main(java.lang.String…);
Code:
0: new #2 // class Dep
3: dup
4: invokespecial #3 // Method Dep.”<init>”:()V
7: astore_1
8: aload_1
9: bipush 123
11: invokevirtual #4 // Method Dep.setF1:(I)V
14: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream;
17: aload_1
18: invokevirtual #6 // Method Dep.getF1:()I
21: invokevirtual #7 // Method java/io/PrintStream.println:(I)V
24: return
}

The most important are 11th and 18th instructions of main method. Main lookups for methods which use int (I in method signature).

Next let’s compile the Main class with Dep which has f1 of type Integer:

javac -cp depInteger main/Main.java

and use javap on this class:

$ javap -c main/Main.class
Compiled from "Main.java"
public class Main {
  public Main();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

public static void main(java.lang.String…);
Code:
0: new #2 // class Dep
3: dup
4: invokespecial #3 // Method Dep.”<init>”:()V
7: astore_1
8: aload_1
9: bipush 123
11: invokestatic #4 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
14: invokevirtual #5 // Method Dep.setF1:(Ljava/lang/Integer;)V
17: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
20: aload_1
21: invokevirtual #7 // Method Dep.getF1:()Ljava/lang/Integer;
24: invokevirtual #8 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
27: return
}

Now we see the difference. The main method:

  • converts int to Integer in instruction 11th,
  • invokes method setF1 which takes parameter of type Integer (Ljava/lang/Integer;) in instruction 14th,
  • invokes method getF1 which returns Integer in instruction 21st.

These differences do not allow us to use the Main class with Dep without recompilation if we change f1.

How about Groovy?

We have GroovyMain class which do the same as Main class written in Java.

class GroovyMain {
    static void main(String... args) {
        Dep dep = new Dep(f1: 123)
        println(dep.f1)
    }
}

We will compile GroovyMain class only with Dep which uses int:

$ groovyc -cp lib/groovy-all-2.4.5.jar:depInt -d main main/GroovyMain.groovy

It runs great as expected with int:

$ java -cp lib/groovy-all-2.4.5.jar:depInt:main GroovyMain
123

but with Integer… It works the same!

$ java -cp lib/groovy-all-2.4.5.jar:depInteger:main GroovyMain
123

Groovy is immune to such change.

With CompileStatic

But what if we compile groovy with CompileStatic annotation? This annotation instructs groovy compiler to compile class with type checking and should produce bytecode similar to javac output.

GroovyMainCompileStatic class is GroovyMain class with only CompileStatic annotation:

import groovy.transform.CompileStatic

@CompileStatic
class GroovyMainCompileStatic {
static void main(String… args) {
Dep dep = new Dep(f1: 123)
println(dep.f1)
}
}

When we compile this with Dep with int field:

$ groovyc -cp lib/groovy-all-2.4.5.jar:depInt -d main main/GroovyMainCompileStatic.groovy

then of course it works:

$ java -cp lib/groovy-all-2.4.5.jar:depInt:main GroovyMainCompileStatic
123

but with Dep with Integer field it fails like in Java:

$ java -cp lib/groovy-all-2.4.5.jar:depInteger:main GroovyMainCompileStatic
Exception in thread "main" java.lang.NoSuchMethodError: Dep.setF1(I)V
    at GroovyMainCompileStatic.main(GroovyMainCompileStatic.groovy:6)

Conclusion

Change from primitive to its wrapped java type is not compatible change. Bytecode which uses dependent class assumes that there will be method which consumes or returns e. g. int and cannot deal with the same class which provides such method with Integer in place of int.

Groovy is much more flexible and could handle it, but only if we do not use CompileStatic annotation.

The source code is available here.

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Log4j and MDC in Grails

Log4j provides very useful feature: MDC - mapped diagnostic context. It can be used to store data in context of current thread. It may sound scary a bit but idea is simple.

My post is based on post http://burtbeckwith.com/blog/?p=521 from Burt Beckwith's excellent blog, it's definitely worth checking if you are interested in Grails.

Short background story...


Suppose we want to do logging our brand new shopping system and we want to have in each log customer's shopping basket number. And our system can be used at once by many users who can perform many transactions, actions like adding items and so on. How can we achieve that? Of course we can add basket number in every place where we do some logging but this task would be boring and error-prone. 

Instead of this we can use MDC to store variable with basket number in map. 

In fact MDC can be treated as map of custom values for current thread that can be used by logger. 


How to do that with Grails?


Using MDC with Grails is quite simple. All we need to do is to create our own custom filter which works for given urls and puts our data in MDC.

Filters in Grails are classes in directory grails-app/conf/* which names end with *Filters.groovy postfix. We can create this class manually or use Grails command: 
grails create-filters info.rnowak.App.Basket

In result class named BasketFilters will be created in grails-app/conf/info/rnowak/UberApp.

Initially filter class looks a little bit empty:
class BasketFilters {
def filters = {
all(controller:'*', action:'*') {
before = {

}
after = { Map model ->

}
afterView = { Exception e ->

}
}
}
}
All we need to do is fill empty closures, modify filter properties and put some data into MDC.

all is the general name of our filter, as class BasketFilters (plural!) can contain many various filters. You can name it whatever you want, for this post let assume it will be named basketFilter

Another thing is change of filter parameters. According to official documentation (link) we can customize our filter in many ways. You can specify controller to be filtered, its actions, filtered urls and so on. In our example you can stay with default option where filter is applied to every action of every controller. If you are interested in filtering only some urls, use uri parameter with expression describing desired urls to be filtered.

Three closures that are already defined in template have their function and they are started in these conditions:

  • before - as name says, it is executed before filtered action takes place
  • after - similarly, it is called after the action
  • afterView - called after rendering of the actions view
Ok, so now we know what are these mysterious methods and when they are called. But what can be done within them? In official Grails docs (link again) under section 7.6.3 there is a list of properties that are available to use in filter.

With that knowledge, we can proceed to implementing filter.

Putting something into MDC in filter


What we want to do is quite easy: we want to retrieve basket number from parameters and put it into MDC in our filter:
class BasketFilters {
def filters = {
basketFilter(controller:'*', action:'*') {
before = {
MDC.put("basketNumber", params.basketNumber ?: "")
}
after = { Map model ->
MDC.remove("basketNumber")
}
}
}
}

We retrieve basket number from Grails params map and then we put in map under specified key ("basketNumber" in this case), which will be later used in logger conversion pattern. It is important to remove custom value after processing of action to avoid leaks.

So we are putting something into MDC. But how make use of it in logs?


We can refer to custom data in MDC in conversion patter using syntax: %X{key}, where key is our key we used in filter to put data, like:
def conversionPattern = "%d{yyyy-MM-dd HH:mm:ss} %-5p %t [%c{1}] %X{basketNumber} - %m%n"


And that's it :) We've put custom data in log4j MDC and successfully used it in logs to display interesting values.