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Both The Elements of Java Style [Vermeulen 2000] and the JPL Java Coding Standard [Havelund 2010] require that the dependency structure of a package

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must never contain cycles

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; that is, it must be representable as a directed acyclic graph (DAG).

There are several pros of eliminating Eliminating cycles between packages has several advantages:

• Testing and maintainability. Cyclic dependencies magnify the repercussions of changes or patches to source code. Reducing the repercussions of changes simplifies testing and improves maintainability. Inability to perform adequate testing because of cyclic dependencies is a frequent source of security vulnerabilities.
• Reusability. Cyclic dependencies between packages require that the packages be released and upgraded in lockstep. This requirement reduces reusability.
• Releases and builds.
• Maintainability: It is preferable to make a change somewhere and limit the repercussions to as few packages as possible (ideally just one) as opposed to having to monitor or refine numerous packages.
• Reusability: When a new version of a package is released, clients who reuse it do not have to test their existing code bases for compatibility with other packages that this particular package depends on. Sometimes, the reusable package evolves only to accommodate the changes to packages that it depends on.
• Releases and builds: Avoiding cycles also helps to steer the development towards toward an environment that fosters modularization. Owners of different packages are also redeemed from relying on other bleeding-edge packages..
• Deployment. Avoiding cyclic dependencies between packages reduces coupling between packages. Reduced coupling reduces the frequency of runtime errors such as ClassNotFoundError. This, in turn, simplifies deploymentDeployment: By resolving the cycles, deployment is simplified, as runtime errors like the infamous ClassNotFoundError, are reduced to a minimum by virtue of the toned down coupling between packages.

Noncompliant Code Example

This noncompliant code snippet features two different example contains packages named Account account and User user that consist of the classes AccountClass and UserClass, AccountHolder, User, and UserDetails respectively. The class UserClass UserDetails extends from AccountClass as one account can have many users or owners. AccountClass depends upon a few utility methods defined in UserClass and must declare its instance AccountHolder because a user is a kind of account holder. The class AccountHolder depends on a nonstatic utility method defined in the User class. Likewise, the UserClass UserDetails depends on AccountClass but instead chooses to extend from it. This is one recipe for a circular dependency AccountHolder by extending it.

package Accountaccount;
import user.User.*;
  class AccountClass {
    private UserClass uc;;
public class AccountHolder {

  private User user;
  public void setUser(User newUser) {user = newUser;}
 
  synchronized void depositFunds(String username, double amount) {
    // Use a utility method of User to check whether username exists
    if (user.exists(username)) { 
      // Deposit the amount
    } 
  }

  protected double getBalance(String accountNumber) { 
    // Return the account balance 
    return 1.0; 
  }
}

package user;
import account.AccountHolder;
public class UserDetails extends AccountHolder {
  public synchronized double getUserBalance(String accountNumber) {
    // uses a class defined in package User
    protectedUse a method of AccountHolder to get the account balance 
    return getBalance(accountNumber); 
  }
}
 
public class User {
  public boolean exists(String username) { 
    // Check whether user exists 
    return true; // Exists
  }
}

Compliant Solution

The tight coupling between the classes in the two packages can be weakened by introducing an interface called BankApplication in a third package, bank. The cyclic package dependency is eliminated by ensuring that the AccountHolder does not depend on User but instead relies on the interface by importing the bank package (and not by implementing the interface).

In this compliant solution, such functionality is achieved by adding a parameter of the interface type BankApplication to the depositFunds() method. This solution gives the AccountHolder a solid contract to bank on. Additionally, UserDetails implements the interface and provides concrete implementations of the methods while at the same time inheriting the other methods from AccountHolder.

package bank;
public interface BankApplication {   
  void depositFunds(BankApplication ba, String username, double amount);
  double getBalance(String accountNumber);
  double getUserBalance(String accountNumber);
  boolean exists(String username); 
}

package account;
import bank.BankApplication;  // Import from a third package
class AccountHolder  {
  private BankApplication ba;
  public void setBankApplication(BankApplication newBA) {
    ba = newBA;
  }

  public synchronized void depositFunds(BankApplication ba, 
      String username, double amount) {	
      // useUse a utility method of UserClass UserDetails 
    // to check ifwhether username exists
     if if(ucba.exists(username)) { 
        // depositDeposit the amount
      } 
   } }
  public  protected double getBalance(String accountNumber) { 
    //* returnReturn the account balance */ }
   
    return 1.0; 
  }   
}

package Useruser;
import Accountaccount.*;
 AccountHolder; // One-way dependency
import bank.BankApplication;  // Import from a third package
public class UserClassUserDetails extends AccountClassAccountHolder {
       protectedimplements BankApplication {
  public synchronized double getUserBalance(
      String accountNumber) {
      // useUse a method of AccountClassAccountHolder to get the account balance 
      return getBalance(accountNumber); 
    }
    public boolean exists(String username) { 
    //* checkCheck whether user exists*/ }
    return true; 
  }

Compliant Solution

TODO

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Risk Assessment

}  

The interface BankApplication appears to contain superfluous methods such as depositFunds() and getBalance(). These methods are present so that if the subclass overrides them, the superclass retains the capability of internally invoking the subclass's methods polymorphically (for example, calling ba.getBalance() with an overridden implementation of the method in UserDetails). One consequence of this solution is that methods declared in the interface are required to be public in the classes that define them.

Applicability

Cyclic dependencies between packages can result in Acyclic dependencies between packages can lead to fragile builds. A security vulnerability in any a package will can easily percolate to several other packages.

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Automated Detection

References

\[[Martin 96|AA. Java References#Martin 96]\] 
\[[Knoernschild 01|AA. Java References#Knoernschild 01]\] Chapter 1: "OO Principles and Patterns, 1.2.5 Acyclic Dependencies Principle"

Bibliography

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Image Added Image Added Image AddedMSC05-J. Make sensitive classes noncloneable      49. Miscellaneous (MSC)      MSC30-J. Generate truly random numbers