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Recall that immutability offers several benefits such as thread-safety, prevention against inadvertent modification of fields and malicious tampering. Class invariants and state are always consistent with the requirements and no defensive copying is necessary. Sometimes howeverHowever, sometimes it is not possible to make sensitive classes immutable. Fortunately, there is a mechanism that allows code to expose mutable classes to untrusted code by granting read-only access. This is largely achieved through unmodifiable wrappers. One For example is of , in the Collection classes wherein a set of wrappers allow clients to observe an unmodifiable view of the particular Collection object.

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class Modifiable {	
  private List<Integer> llist = new ArrayList<Integer>();

  public void listIt() {
    doSomethingaddSomething(llist);
    // ...
  }

  private void doSomethingaddSomething(Collection<Integer> collection) {
    collection.add(1); 
  }
}

...

// ...
doSomethingpublic void listIt() {
  addSomething(Collections.unmodifiableCollection(list));
  // ...
}

private void doSomethingaddSomething(Collection<Integer> collection) {
  collection.add(1); // throws java.lang.UnsupportedOperationException
} 

It should be noted that objects present within the Collection may not be thread-safe, making them mutable in multithreaded contexts. Consider for example, an ArrayList of ArrayLists wherein the contained ArrayList is still susceptible to modification.

Noncompliant Code Example

This noncompliant code example shows an interface MutableInterface which declares an accessor method and a mutator method. This class does not expose an unmodifiable view to implementing clients.

interface MutableInterface {
  int[] getArray(); // accessor
  void setArray(int[] i); //mutator
}

class SensitiveMutable implements MutableInterface {
  int[] array = new int[10]; // mutable array

  public int[] getArray() {
    return array.clone(); 
  }

  public void setArray(int[] i) {
    array = i;
  }
}

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In general, sensitive classes can be transformed into safe-view objects by implementing all the methods defined by the core interface, including the mutator methods. In this case, the difference is that the mutators need to throw an UnsupportedOperationException so that clients cannot carry out operations that affect the immutability property of the object.

In this This compliant solution , constructs an UnmodifiableSensitiveMutable is constructed by extending the class SensitiveMutable. An interface UnmodifiableInterface consists of the method unmodifiableView() which accepts a SensitiveMutable object as the sole parameter. It returns an equivalent object that is a subtype of the same class, and which is unmodifiable. An exception is thrown if the caller attempts to use the mutator method on the returned object. This object can be passed to untrusted code as required.

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Failure to provide an unmodifiable safe-view of a sensitive mutable objects object to untrusted code can lead to malicious tampering and corruption of the object.

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