InvocationLocal<T>

InvocationLocal.java

/**
 * Provides method-invocation-scope variables.
 * <p/>
 * Each invocation of {@link #get()} within same method (including nested calls) will yield value previously set by
 * {@link #set(Object)}. Once {@link #set(Object)} region ends, value is rolled back to the default one (provided
 * during {@link InvocationLocal} instance construction).
 * <p/>
 * Implementation is thread-safe.
 *
 * @author <a href="mailto:sshyiko@cogniance.com">sshyiko</a>
 */
public class InvocationLocal<T> {

    private static final int DEFAULT_STACK_SHIFT = 4;

    private ThreadLocal<State> threadLocalHolder = new ThreadLocal<State>();
    private final int stackShift;

    public InvocationLocal(final T defaultValue) {
        this(new DefaultValueProvider<T>() {
            
            @Override
            public T get() {
                return defaultValue;
            }
        }, 0);
    }

    public InvocationLocal(final T defaultValue, final int stackShift) {
        this(new DefaultValueProvider<T>() {

            @Override
            public T get() {
                return defaultValue;
            }
        }, stackShift);
    }

    public InvocationLocal(final DefaultValueProvider<T> defaultValueProvider) {
        this(defaultValueProvider, 0);
    }

    public InvocationLocal(final DefaultValueProvider<T> defaultValueProvider, final int stackShift) {
        threadLocalHolder = new ThreadLocal<State>() {

            private final String[] EMPTY_STACK_TRACE = new String[0];

            @Override
            protected State initialValue() {
                return new State(EMPTY_STACK_TRACE, defaultValueProvider.get());
            }
        };
        this.stackShift = Math.max(stackShift + DEFAULT_STACK_SHIFT, DEFAULT_STACK_SHIFT);
    }

    public T get() {
        State state = threadLocalHolder.get();
        if (!isNestedInvocation(state.getStackTrace(), getStackTrace())) {
            threadLocalHolder.remove();
            state = threadLocalHolder.get();
        }
        return state.getValue();
    }
    
    public T getIgnoringScope() {
        return threadLocalHolder.get().getValue();
    }

    public void set(T value) {
        threadLocalHolder.set(new State(getStackTrace(), value));
    }

    public void updateScope() {
        threadLocalHolder.get().setStackTrace(getStackTrace());
    }
    
    private boolean isNestedInvocation(String[] stackTrace, String[] nestedStackTrace) {
        int stackTraceLength = stackTrace.length;
        if (stackTraceLength > nestedStackTrace.length) {
            return false;
        }
        for (int i = 0; i < stackTraceLength; i++) {
            if (!stackTrace[i].equals(nestedStackTrace[i])) {
                return false;
            }
        }
        return true;
    }

    private String[] getStackTrace() {
        StackTraceElement[] stackTrace = Thread.currentThread().getStackTrace();
        String[] result = new String[stackTrace.length - stackShift];
        int resultIndex = result.length - 1, stackIndex = 0;
        for (StackTraceElement stackTraceElement : stackTrace) {
            if (stackIndex++ < stackShift) {
                continue;
            }
            result[resultIndex--] = stackTraceElement.toString();
        }
        return result;
    }
    
    private class State {
        
        private String[] stackTrace;
        private T value;

        private State(String[] stackTrace, T value) {
            this.stackTrace = stackTrace;
            this.value = value;
        }

        public String[] getStackTrace() {
            return stackTrace;
        }

        public void setStackTrace(String[] stackTrace) {
            this.stackTrace = stackTrace;
        }

        public T getValue() {
            return value;
        }

        public void setValue(T value) {
            this.value = value;
        }
    }
    
    public interface DefaultValueProvider<T> {
        
        T get();
    }
}

@Test
public void testNestedCall() {
    InvocationLocal<Integer> invocationScopeLocal = new InvocationLocal<Integer>(1);
    runNested(invocationScopeLocal);
    assertEqualsNested(invocationScopeLocal, 1);
}

private void runNested(InvocationLocal<Integer> invocationScopeLocal) {
    invocationScopeLocal.set(2);
    assertEquals(2, (int) invocationScopeLocal.get());
    assertEqualsNested(invocationScopeLocal, 2);
}

private void assertEqualsNested(InvocationLocal<Integer> invocationScopeLocal, int value) {
    assertEquals(value, (int) invocationScopeLocal.get());
}