How to use shouldBeAbleToCopyFromRealObjectToRealObject method of org.mockito.internal.util.reflection.FieldReaderTest class

Adding RETURNS_DEEP_STUBS did the trick:

A a = Mockito.mock(A.class, Mockito.RETURNS_DEEP_STUBS);

There is a type-safe way: use ArgumentMatchers.any() and qualify it with the type:

ArgumentMatchers.<AsyncCallback<ResponseX>>any()

Like Rob said, if what you want is to mock the connection returned from the URL, you can extend URLStreamHandler. For instance, with mockito:

final URLConnection mockUrlCon = mock(URLConnection.class);

ByteArrayInputStream is = new ByteArrayInputStream(
        "<myList></myList>".getBytes("UTF-8"));
doReturn(is).when(mockUrlCon).getInputStream();

//make getLastModified() return first 10, then 11
when(mockUrlCon.getLastModified()).thenReturn((Long)10L, (Long)11L);

URLStreamHandler stubUrlHandler = new URLStreamHandler() {
    @Override
     protected URLConnection openConnection(URL u) throws IOException {
        return mockUrlCon;
     }            
};
URL url = new URL("foo", "bar", 99, "/foobar", stubUrlHandler);
doReturn(url).when(mockClassloader).getResource("pseudo-xml-path");

InOrder helps you to do that.

ServiceClassA firstMock = mock(ServiceClassA.class);
ServiceClassB secondMock = mock(ServiceClassB.class);

Mockito.doNothing().when(firstMock).methodOne();   
Mockito.doNothing().when(secondMock).methodTwo();  

//create inOrder object passing any mocks that need to be verified in order
InOrder inOrder = inOrder(firstMock, secondMock);

//following will make sure that firstMock was called before secondMock
inOrder.verify(firstMock).methodOne();
inOrder.verify(secondMock).methodTwo();

I could make your unit test pass by modifying 2 things in your code:

1. Mock mockServerSocket.accept() properly

Up to now, you mock mockServerSocket.accept() too early because mockTestClientSocket has not yet been set so it will return null, you need to set it first so your code should rather be:

mockServerSocket = mock(ServerSocket.class);
// Set it first
mockTestClientSocket = mock(Socket.class);

try {
    // Then mock it
    when(mockServerSocket.accept()).thenReturn(mockTestClientSocket);
} catch (IOException e) {
    fail(e.getMessage());
}
...

2. Synchronize your threads properly

As you launch a dedicated thread to manage your client socket, you need to synchronize your threads in order to make sure that your message will be ready to be read.

To do this you could:

1. Simplify your code by calling reader.readLine() but it is a blocking approach since your current thread will wait as long as it is needed for the other thread to write the line (the message here).

The code could be:

BufferedReader reader = ...

String receivedMessage = reader.readLine();
assertEquals("Hello, world!", receivedMessage);
mockTestClientSocket.close();
connection.closeSocket();

2. Set a value for TIMEOUT_TIME big enough even exaggeratedly big to make sure that the other thread will be ready, so for example as it is a value in nanoseconds, you could set it to 30 seconds so to 30_000_000_000L. If you don't set a value big enough, your test could be unstable in slow and/or overloaded and/or shared system such as server used for continuous integration.

I recommend to use approach from article: https://github.com/mockito/mockito/wiki/Mocking-Object-Creation

You need to change little bit a class, applying approach for mocking from article in following way:

AwsHelper

public class AwsHelper {

    private AmazonSQS sqs;

    public void sendMessageToQueue(String message) {
        sqs = defaultClient();

        SendMessageRequest sendMessageRequest = new SendMessageRequest();
        sendMessageRequest.setQueueUrl("");
        sendMessageRequest.setMessageBody(message);
        sendMessageRequest.setMessageGroupId("");

        sqs.sendMessage(sendMessageRequest);
    }

    protected AmazonSQS defaultClient() {
        return AmazonSQSClientBuilder.defaultClient();
    }
}

AwsSQSReferralsUtilTest

public class AwsSQSReferralsUtilTest {

    @Spy
    private AwsHelper awsHelper;

    @Mock
    private AmazonSQS sqs;

    @BeforeClass
    public void setUp() {
        MockitoAnnotations.initMocks(this);
    }
    
    @AfterMethod
    public void afterEachMethod() {
        Mockito.reset(awsHelper);
    }

    @Test
    public void shouldSendMessage() {
        //mocking object creation
        doReturn(sqs).when(awsHelper).defaultClient();

        when(sqs.sendMessage(any(SendMessageRequest.class))).thenReturn(new SendMessageRequest());
        awsHelper.sendMessageToQueue("");
    }

}

You simply forgot to add the dependencies to your project which are according to the pom file:

<dependency>
  <groupId>net.bytebuddy</groupId>
  <artifactId>byte-buddy</artifactId>
  <version>1.3.16</version>
  <scope>runtime</scope>
</dependency>
<dependency>
  <groupId>org.objenesis</groupId>
  <artifactId>objenesis</artifactId>
  <version>2.1</version>
  <scope>runtime</scope>
</dependency>

In other words you need to add byte-buddy 1.3.16 and objenesis 2.1 to your project too.

More details here

Make sure powermockito and mockito versions are aligned as in this versions chart - MockitoUsage#supported-versions,

Mockito                     | PowerMock
------------------------------------------------------------------------------
2.0.0-beta - 2.0.42-beta    |   1.6.5+
------------------------------------------------------------------------------
1.10.19                     |   1.6.4
1.10.8 - 1.10.x             |   1.6.2+
1.9.5-rc1 - 1.9.5           |   1.5.0 - 1.5.6
1.9.0-rc1 & 1.9.0           |   1.4.10 - 1.4.12
1.8.5                       |   1.3.9 - 1.4.9
1.8.4                       |   1.3.7 & 1.3.8
1.8.3                       |   1.3.6
1.8.1 & 1.8.2               |   1.3.5
1.8                         |   1.3
1.7                         |   1.2.5

Easy way to find mockito and powermock-mockito version using maven is,

mvn dependency:tree | grep mockito
[INFO] |  \- org.mockito:mockito-core:jar:1.8.5:compile
[INFO] +- org.mockito:mockito-all:jar:1.8.5:compile
[INFO] +- org.powermock:powermock-api-mockito:jar:1.4.9:compile

Problem could be the conflicting versions of mockito in the application and the one that powermockito uses, conflicting as below in my case where I'm using powermock 1.6.5 which does not support mockito 1.8.5

mvn clean dependency:tree | grep mockito
[INFO] +- org.mockito:mockito-all:jar:1.8.5:compile

[INFO] \- org.powermock:powermock-api-mockito:jar:1.6.5:compile
[INFO]    +- org.mockito:mockito-core:jar:1.10.19:compile
[INFO]    \- org.powermock:powermock-api-mockito-common:jar:1.6.5:compile

All warnings go away when alternative way of mock specification is used:

    ScheduledFuture<?> future = Mockito.mock(ScheduledFuture.class);
    Mockito.doReturn(future).when(executor).schedule(
        Mockito.any(Runnable.class),
        Mockito.anyLong(),
        Mockito.any(TimeUnit.class)
    );