Initializer Thread Safety Example 1

An example of a potential gotcha when using the Initializer in jrugged.

package org.fishwife.jrugged;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.fail;

import org.junit.Test;

/**
 * Starting a thread within an object constructor is an anti-pattern. See Java Concurrency In Practice. The Java memory
 * model does not guarantee that the fields of an object will contain correct references until the constructor
 * returns. But the Initializer _requires_ you to publish a reference to the Initializable _during_ object construction
 * (before the constructor returns). This means Initializer's background thread can see the wrong references. Calling
 * the initializer as the last method in the constructor will not help because:
 *
 * 1. The java memory model does not guarantee that an object's final fields are initialized until after the constructor
 * completes.
 *
 * 2. A sub-class can perform may have more constructor logic after the initializer is invoked.
 *
 * Rather than starting threads in the constructor, the recommended practice is to create
 * a separate initialize() method that is invoked on an object _after_ it is fully constructed, meaning all the java
 * memory model guarantees are in place. The initialize() method can then delegate to Initializer . . . within certain
 * constraints as shown by InitializerThreadSafetyExample2.
 *
 * If you are willing to deal with the potential exceptions/nulls, it is possible to wait (as this example shows) 
 * for the object construction to be complete.
 *
 * This test should print the following (or something similar):
 *
 * Found null value for final field!
 * Found null value for final field!
 * Found null value for final field!
 * Found null value for final field!
 * Found null value for final field!
 * Found null value for final field!
 * Found null value for final field!
 * Found null value for final field!
 * Found null value for final field!
 * Found null value for final field!
 * Constructor finished so populating values
 * Set status to Status.UP
 */
public class TestInitializerThreadSafety1 {
        @Test(timeout = 1000)
        public void threadSafety() throws Exception {
                Service testable = new Service();

                while (Status.INIT == testable.status) { }

                for (int valIdx = 0; valIdx < testable.values.length; valIdx++ ) {
                        assertEquals(Integer.toString(valIdx), valIdx, testable.values[valIdx]);
                }
        }

        private static class Service implements Initializable {

                private volatile Status status = Status.INIT;

                /**
                 * The java memory model guarantees final references are set only after the constructor completes. But Initializer
                 * requires the "this" reference to escape _during_ object construction. This means the background thread is given a
                 * reference to a partially constructed object.
                 */
                private final long[] values;

                private Initializer initializer;

                public Service() {

                        initializer = new Initializer(this);

                        initializer.setRetryMillis(10);
                        initializer.initialize();

                        try {
                                Thread.sleep(100);
                        } catch (InterruptedException e) {
                                fail("Should not happen");
                        }

                        values = new long[16834];
                }

                @Override
                public void tryInit() throws Exception {
                        if (null == values) {

                                System.out.println("Found null value for final field!");

                                throw new NullPointerException();

                        } else {

                                System.out.println("Constructor finished so populating values");

                                for (int i = 0; i < values.length; i++) {
                                        values[i] = i;
                                }
                        }
                }

                @Override
                public void afterInit() {
                        status = Status.UP;

                        System.out.println("Set status to Status.UP");
                }
        }
}