VNA04-J
2.5VNA04-J. Ensure that calls to chained methods are atomic
Method chaining is a convenience mechanism that allows multiple method invocations on the same object to occur in a single statement. A method-chaining implementation consists of a series of methods that return the this reference. This implementation allows a caller to invoke methods in a chain by performing the next method invocation on the return value of the previous method in the chain.
While the methods used in method chaining can be atomic, the chain they comprise is inherently non-atomic. Consequently, methods that are involved in method chaining should not be invoked concurrently unless the caller provides sufficient locking as illustrated in guideline “VNA03-J. Do not assume that a group of calls to independently atomic methods is atomic” on page 23.
2.5.1Noncompliant Code Example
Method chaining is a useful design pattern for building an object and setting its optional fields. A class that supports method chaining provides several setter methods that each return the this reference. However, if accessed concurrently, a thread may observe shared fields to contain inconsistent values. This noncompliant code example shows the JavaBeans pattern, which is not thread-safe.
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;
}
}
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// Client code:
private final USCurrency currency = new USCurrency(); // ...
new Thread(new Runnable() { @Override public void run() {
currency.setQuarters(1).setDimes(1);
}
}).start();
new Thread(new Runnable() { @Override public void run() {
currency.setQuarters(2).setDimes(2);
}
}).start();
The JavaBeans pattern uses a no-argument constructor and a series of parallel setter methods to build an object. This pattern is not thread-safe and can lead to inconsistent object state if the object is modified concurrently. In this noncompliant code example, the client constructs a USCurrency object and starts two threads that use method chaining to set the optional values of the USCurrency object. This example code might result in the USCurrency instance being left in an inconsistent state, for example, with two quarters and one dime, or one quarter and two dimes.
2.5.2Compliant Solution
This compliant solution uses a variant of the Builder pattern [Gamma 1995] suggested by Bloch [Bloch 2008] to ensure the 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;
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public static Builder newInstance() { return new Builder();
}
private 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:
private volatile USCurrency currency; // ...
new Thread(new Runnable() { @Override public void run() {
currency = USCurrency.Builder.newInstance().setQuarters(1).setDimes(1).build();
}
}).start();
new Thread(new Runnable() { @Override public void run() {
currency = USCurrency.Builder.newInstance().setQuarters(2).setDimes(2).build();
}
}).start();
The Builder.newInstance() factory method is called with any required arguments to obtain a Builder instance. The optional parameters are set using the setter methods of the builder. The
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object construction concludes with the invocation of the build() method. This pattern makes the USCurrency class immutable and, consequently, thread-safe.
Note that the currency field cannot be declared final because it is assigned a new immutable object. It is, however, declared volatile in compliance with guideline “VNA01-J. Ensure visibility of shared references to immutable objects” on page 13.
If input needs to be validated, ensure that the values are defensively copied prior to validation (see guideline “FIO00-J. Defensively copy mutable inputs and mutable internal components2” for more information). The builder class does not violate guideline “SCP03-J. Do not expose sensitive private members of the outer class from within a nested class2” because it maintains a copy of the variables defined in the scope of the containing class. The private members within the nested class take precedence, and as a result, do not break encapsulation.
2.5.3Risk Assessment
Using method chaining in multithreaded environments without performing external locking can lead to nondeterministic behavior.
Guideline |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
VNA04- J |
low |
probable |
medium |
P4 |
L3 |
2.5.4References
[Bloch 2008] |
Item 2: “Consider a builder when faced with many constructor parameters” |
[Sun 2009b] |
|
2 |
This guideline is described at https://www.securecoding.cert.org/confluence/display/java/. |
|
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