Hellokindly consider this example Class: public with sharing class C {
public C() {
ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.INFO, 'Zero-constructor called'));
}
public C(C controller) {
ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.INFO, 'Single-argument C called'));
}
}
If your page written like this:
Then the zero-parameter constructor will be called.<apex:page controller="wrapperAccountOpportunity" ...
If your page is written like this:
Then the constructor with the single parameter ApexPages.StandardController will be called.If you want to call fetchQuery from both constructors, you'll have to call it from both constructors. Only one constructor will ever be called per controller or extension.And, given the following page:<apex:page standardController="Account" extensions="wrapperAccountOpportunity" ...
<apex:page controller="C" extensions="C">
<apex:messages />
</apex:page>
You may expect both constructors to be called, but instead, you'll find only the zero-length (default) constructor is called, since the class is referenced twice. In this sense, classes that become controllers are actually singletons in Visualforce. The controller version of the class is only instantiated once; it can still be constructed normally multiple times within the code itself, but Visualforce will only instantiate one automatically.
Hope its clear.
