java - Synthetic accessor method warning

Question

I've made some new warning settings in eclipse. With these new settings I'm facing a strange warning. After reading I got to know what it is but couldn't find a way to remove it.

Here is my problem with the sample code

public class Test {
    private String testString;

    public void performAction() {

        new Thread( new Runnable() {
            @Override
            public void run() {
                testString = "initialize"; // **
            }
        });
    }
}

The line with ** gives me a warning in eclipse

Read access to enclosing field Test.testString is emulated by a synthetic accessor method. 
Increasing its visibility will improve your performance.

Problem is, I don't want to change the access modifier of testString. Also, don't want to create a getter for it.

What change should be done?

---

More descriptive example 

public class Synthetic
{
    private JButton testButton;

    public Synthetic()
    {
        testButton = new JButton("Run");
        testButton.addActionListener(new ActionListener() {
                public void actionPerformed(ActionEvent ae)
                {
                    /* Sample code */
                    if( testButton.getText().equals("Pause") ) {
                        resetButton(); // **    
                    } else if( testButton.getText().equals("Run") ) {
                        testButton.setText("Pause"); // **  
                    }

                }
            }
        );
    }

    public void reset() {
        //Some operations
        resetButton();
    }

    private void resetButton() {
        testButton.setText("Run");
    }
}

Lines with ** gives me the same warning.

Answer 1

What is a "synthetic" method?

Starting from the Method class (and it's parent, Member) we learn that synthetic members are "introduced by the compiler", and that JLS §13.1 will tell us more. It notes:

A construct emitted by a Java compiler must be marked as synthetic if it does not correspond to a construct declared explicitly or implicitly in source code

Since this section is discussing binary compatibility the JVMS is also worth referencing, and JVMS §4.7.8 adds a little more context:

A class member that does not appear in the source code must be marked using a Synthetic attribute, or else it must have its ACC_SYNTHETIC flag set. The only exceptions to this requirement are compiler-generated methods which are not considered implementation artifacts....

The Synthetic attribute was introduced in JDK 1.1 to support nested classes and interfaces.

In other words, "synthetic" methods are an implementation artifact that the Java compiler introduces in order to support language features that the JVM itself does not support.

What's the problem?

You're running into one such case; you're attempting to access a private field of a class from an anonymous inner class. The Java language permits this but the JVM doesn't support it, and so the Java compiler generates a synthetic method that exposes the private field to the inner class. This is safe because the compiler doesn't allow any other classes to call this method, however it does introduce two (small) issues:

1. Additional methods are being declared. This shouldn't be a problem for the vast majority of use cases, but if you're working in a constrained environment like Android and are generating a lot of these synthetic methods you may run into issues.
2. Access to this field is done indirectly through the synthetic method, rather than directly. This too shouldn't be a problem except for highly performance-sensitive use cases. If you wouldn't want to use a getter method here for performance reasons, you wouldn't want a synthetic getter method either. This is rarely an issue in practice.

In short, they're really not bad. Unless you have a concrete reason to avoid synthetic methods (i.e. you've determined conclusively they are a bottleneck in your application) you should just let the compiler generate them as it sees fit. Consider turning off the Eclipse warning if it's going to bother you.

What should I do about them?

If you really want to prevent the compiler from generating synthetic methods you have a couple of options:

Option 1: Change the permissions

Package-private or protected fields are accessible to inner classes directly. Especially for something like a Swing application this ought to be fine. But you say you want to avoid this, so on we go.

Option 2: Create a getter

Leave your fields alone, but explicitly create a protected or public getter, and use that instead. This is essentially what the compiler ends up doing for you automatically, but now you have direct control over the method's behavior.

Option 3: Use a local variable and share the reference with both classes

This is more code but it's my personal favorite since you're making the relationship between the inner and outer class explicit.

public Synthetic() {
  // Create final local instance - will be reachable by the inner class
  final JButton testButton = new JButton("Run");
  testButton.addActionListener(
      new ActionListener() {
        public void actionPerformed(ActionEvent ae) {
          /* Sample code */
          if( testButton.getText().equals("Pause") ) {
            resetButton();
          } else if( testButton.getText().equals("Run") ) {
            testButton.setText("Pause");
          }
        }
      });
  // associate same instance with outer class - this.testButton can be final too
  this.testButton = testButton;
}

This isn't always actually what you want to do. For instance if testButton can change to point to a different object later, you'll need to rebuild your ActionListener again (though that's also nicely more explicit, so arguably this is a feature), but I consider it the option that most clearly demonstrates its intent.

---

Aside on thread-safety

Your example Test class is not thread-safe - testString is being set in a separate Thread but you're not synchronizing on that assignment. Marking testString as volatile would be sufficient to ensure all threads see the update. The Synthetic example doesn't have this problem since testButton is only set in the constructor, but since that's the case it would be advisable to mark testButton as final.