CON30-J. Do not use method chaining implementations in a multi-threaded environment

Method chaining is a convenience mechanism that allows multiple method invocations on the same object to occur in a single statement. Classes that support method chaining provide several setter methods that return the this reference.

Method chaining should not be used in a multi-threaded environment because chained invocations of a set of methods is non-atomic and consequently, noncompliant with [CON07-J. Do not assume that a group of calls to independently atomic methods is atomic].

Noncompliant Code Example

Method chaining is useful for building an object and setting its optional fields. However, in a multi-threaded environment, a thread may observe a shared field to contain inconsistent values. This noncompliant code example shows the Javabeans pattern which is not safe for multithreaded use.

final class USCurrency {
  // Change requested, denomination (optional fields)
  private int quarters = 0;
  private int dimes = 0;
  private int nickels = 0;
  private int pennies = 0;

  public USCurrency() {}

  // Setter methods 
  public USCurrency setQuarters(int quantity) { 
    quarters = quantity; 
    return this;
  } 
  public USCurrency setDimes(int quantity) { 
    dimes = quantity; 
    return this;
  }
  public USCurrency setNickels(int quantity) { 
    nickels = quantity;
    return this;
  }
  public USCurrency setPennies(int quantity) { 
    pennies = quantity;
    return this;
  }
}

// ...

// Client code:
new Thread(new Runnable() {
  public void run() {    
    USCurrency currency = new USCurrency().currency.setQuarters(1).setDimes(1);
  }
}).start();
new Thread(new Runnable() {
  public void run() {
    USCurrency currency = new USCurrency().currency.setQuarters(2).setDimes(2);
  }
}).start();

The Javabeans pattern uses a no-argument constructor along with a series of parallel setter methods to build an object. This pattern is not thread-safe and can lead to inconsistent object state. In this example, a client thread that constructs a USCurrency object may find the object to contain one quarter and two dimes.

Compliant Solution

This compliant solution uses the variant of the Builder pattern [[Gamma 95]] suggested by Bloch [[Bloch 08]] to ensure thread safety and atomicity of object creation.

final class USCurrency {
  private final int quarters;
  private final int dimes;
  private final int nickels;
  private final int pennies;

  public USCurrency(Builder builder) {
    this.quarters = builder.quarters;
    this.dimes = builder.dimes;
    this.nickels = builder.nickels;
    this.pennies = builder.pennies;
  }

  // Static class member 
  public static class Builder {
    private int quarters = 0;
    private int dimes = 0;
    private int nickels = 0;
    private int pennies = 0;

    public Builder() {}   

    // Setter methods 
    public Builder setQuarters(int quantity) { 
      this.quarters = quantity; 
      return this;
    } 
    public Builder setDimes(int quantity) { 
      this.dimes = quantity; 
      return this;
    }
    public Builder setNickels(int quantity) { 
      this.nickels = quantity;
      return this;
    }
    public Builder setPennies(int quantity) { 
      this.pennies = quantity;
      return this;
    }

    public USCurrency build() {
      return new USCurrency(this);
    }
  }
}

// Client code:
USCurrency currency = USCurrency.Builder().setQuarters(1).setDimes(1).build();
USCurrency currency = USCurrency.Builder().setQuarters(2).setDimes(2).build();

The Builder constructor is called with the required arguments (if any) to obtain a builder object. The optional parameters are set using the setter methods on the builder. The object construction concludes with the invocation of the build() method. This also makes the class Currency immutable, and consequently, thread-safe.

If input needs to be validated, make sure that the values are copied from the builder class to the containing outer class's fields prior to the validation. The builder class does not violate [SCP03-J. Do not expose sensitive private members of the outer class from within a nested class] because it maintains a copy of the variables defined in the scope of the containing class. These take precedence and as a result, do not break encapsulation.

If the number of fields is small, it is better to synchronize the setter methods instead of using this design pattern. But care must be taken to ensure that the setter methods provide the required degree of atomicity. (see [CON07-J. Do not assume that a group of calls to independently atomic methods is atomic] for more information.)

Exceptions

EX1: A class may use method chaining in a multithreaded environment if it sufficiently documents this fact. Client code must externally use some locking to ensure that the method calls are thread-safe.

// This class is not thread-safe. A client must provide locking on any
// USCurrency object in a multi-threaded environment.
final class USCurrency {
  // Change requested, denomination (optional fields)
  private int quarters = 0;
  private int dimes = 0;
  private int nickels = 0;
  private int pennies = 0;

  public USCurrency() {}

  // Setter methods 
  public USCurrency setQuarters(int quantity) { 
    quarters = quantity; 
    return this;
  } 
  public USCurrency setDimes(int quantity) { 
    dimes = quantity; 
    return this;
  }
  public USCurrency setNickels(int quantity) { 
    nickels = quantity;
    return this;
  }
  public USCurrency setPennies(int quantity) { 
    pennies = quantity;
    return this;
  }
}

// ...

// Client code:
final USCurrency currency = new USCurrency();
final Object lock = new Object();
new Thread(new Runnable() {
  public void run() {
    synchronized (lock) {
      currency.setQuarters(1).setDimes(1);
    }
  }
}).start();
new Thread(new Runnable() {
  public void run() {
    synchronized (lock) {
      currency.setQuarters(2).setDimes(2);
    }
  }
}).start();

Risk Assessment

Using implementations of method chaining that are not thread-safe can lead to non-deterministic behavior.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]]
[[Bloch 08]] Item 7, Avoid finalizers

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