Constructor injection

This is the most common way to inject dependency. In this approach, you need to pass the dependent object through a public constructor of a client object. Please note that in case of construction injection, you need to pass all the dependency objects in the constructor of a client object.

Constructor injection can control the order of instantiation and consequently reduce the risk of circular dependency. All mandatory dependencies can be passed through constructor injection.

In our BalanceSheet example, we need to pass two objects in a constructor, because it has two dependencies: one is for fetch data, and the second is for export data types, as per the following snippet:

public class BalanceSheet {

  private IExportData exportDataObj= null;
  private IFetchData fetchDataObj= null;
  
  //All dependencies are injected from client's constructor 
  BalanceSheet(IFetchData fetchData, IExportData exportData){
    this.fetchDataObj = fetchData;
    this.exportDataObj = exportData;
  }
  
  public Object generateBalanceSheet(){
    List<Object[]> dataLst = fetchDataObj.fetchData();
    return exportDataObj.exportData(dataLst);
  }
}

All dependencies are injected from a constructor of a client object. Since constructors are called only once, it's clear that the dependency object will not be changed until the existence of a client object. If a client uses constructor injection, then extending and overriding it would be difficult sometimes.