我建议在测试中更改一些东西。不要模拟S3Object，而要从本地文件构建对象，将值设置为模拟对象是不正确的。
您也应该加入例外状况的测试。
查看下面的代码

import com.amazonaws.AmazonServiceException;
import com.amazonaws.services.s3.AmazonS3Client;
import com.amazonaws.services.s3.model.GetObjectRequest;
import com.amazonaws.services.s3.model.S3Object;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.junit.MockitoJUnitRunner;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.UnsupportedEncodingException;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertThrows;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

@RunWith(MockitoJUnitRunner.class)
public class ExampleServiceTest {

    @Mock
    private AmazonS3Client mockS3Client;

    @Test
    public void getObjectTest() throws UnsupportedEncodingException, FileNotFoundException {
        String testBucketName = "test_bucket", s3Path = "/files/A.json";
        S3Object s3Object = buildS3Object();
        when(mockS3Client.getObject(new GetObjectRequest(testBucketName, s3Path))).thenReturn(s3Object);
        ExampleService exampleService = new ExampleService(mockS3Client);

        exampleService.getObject(testBucketName, s3Path);

        verify(mockS3Client).getObject(new GetObjectRequest(testBucketName, s3Path));
    }

    @Test
    public void ShouldThrowRuntimeExceptionServiceErrorMessageWhenAmazonServiceException() throws FileNotFoundException {
        String testBucketName = "test_bucket", s3Path = "/files/A.json";

        when(mockS3Client.getObject(new GetObjectRequest(testBucketName, s3Path))).thenThrow(AmazonServiceException.class);
        ExampleService exampleService = new ExampleService(mockS3Client);

        Exception exception = assertThrows(RuntimeException.class, () -> {
            exampleService.getObject(testBucketName, s3Path);
        });

        assertEquals("Service error while getting object=/files/A.json in bucket=test_bucket", exception.getMessage());
    }

    private S3Object buildS3Object() throws FileNotFoundException {
        S3Object s3Object = new S3Object();
        s3Object.setObjectContent(new FileInputStream("your_path/src/test/resources/A.json"));
        return s3Object;
    }
}