I was writing this piece of code to understand reflection and encountered one scenario where I couldn't really figure out the reason for the codes' behavior. Hopefully I receive some guidance from the community.
Following is my test model class & here, for every instantiation, I want to know the exact number of instances created during runtime (using reflection)
public final class Model {
private static final Model instance = new Model("Testing");
private static int count = 0;
private String name;
private Model(String name) {
this.name = name;
++count;
}
public static Model getInstance() {
return instance;
}
public static int getInstanceCount() {
return count;
}
public String getName() {
return name;
}
public void doSomething() {
try {
System.out.println("Shh.... I am trying to do something");
Thread.sleep(1000);
System.out.println("Ok! Done.");
return;
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
System.out.println("Oops! I failed in doing your job...");
}
}
The driver code for this scenario is as follows,
public class ReflectionTest {
public static void main(String[] args) throws Exception {
Model.getInstance().doSomething();
System.out.println(Model.getInstanceCount());
Constructor<?>[] constructor = Model.class.getDeclaredConstructors();
for (Constructor<?> aConstructor : constructor) {
aConstructor.setAccessible(true);
Model m = (Model) aConstructor.newInstance("Testing through Reflection");
System.out.println(m.getName());
m.doSomething();
System.out.println(m.getInstanceCount());
//System.out.println(Model.getInstanceCount());
}
}
}
The output for this above piece of code came out to be as follows,
Shh.... I am trying to do something
Ok! Done.
0
Testing through Reflection
Shh.... I am trying to do something
Ok! Done.
1
As you can see, the instance count came out to be 1. I expected it to be as 2.
However, I changed the test model class's constructor as shown below. The datatype of count is now changed to Integer, instead of previously set 'int' .
private Model(String name) {
this.name = name;
if (count == null)
count = 0;
++count;
}
Surprisingly, I get the correct value for the instance count.
Shh.... I am trying to do something
Ok! Done.
1
Testing through Reflection
Shh.... I am trying to do something
Ok! Done.
2
This might be a silly question, but I am not able to ponder on what really happened behind the scenes. I need some guidance from the community on this.
Thanks in advance.
This has nothing to do with reflection.
private static final Model instance = new Model("Testing");
private static int count = 0;
The initializers are executed in order. So:
private static final Model instance = new Model("Testing");
Executing the constructor causes count
to be incremented from 0 to 1, but then:
private static int count = 0;
Sets count back to zero.
Reverse the order of the declarations.
private static int count = 0;
private static final Model instance = new Model("Testing");
Or omit the initializer on count
(its default value is zero anyway).
private static final Model instance = new Model("Testing");
private static int count;
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