Implement Args builder for getObject API (#954)

* Implement Args builder for getObject API

New Argument class: `GetObjectArgs`

Arguments supported:
 - offset (Long)
 - length (Long)
 - ssec (ServerSideEncryptionWithCustomerKey)

Since there are multiple APIs / args classes that need sse as an
argument, introduced an intermediate abstract class `SsecObjectArgs`
that supports the `ssec` argument, and provides common logic related to
it e.g. validations.

Since the return type of the methods that support downloading the
object as a file is `void`, we cannot name it same as `getObject`. So
introduced a new method called `downloadObject` for downloading the
object, which internally uses `getObject`. And created a separate args
class `DownloadObjectArgs` for it.

Also,

- Enhanced the method `FunctionalTest#handleException` to support
logging of the arguments as well.
- Moved the class `ServerSideEncryptionWithCustomerKey` outside of
`ServerSideEncryption` and made it into a standalone class.

* Incorporate review comments

- Change `sse` to `ssec` in javadoc comments on stateObject
- Remove unused static variables from `SsecObjectArgs`

api/src/main/java/io/minio/ComposeSource.java

@@ -29,7 +29,7 @@ public class ComposeSource {
   private Long length;
   private Map<String, String> headerMap;
   private CopyConditions copyConditions;
-  private ServerSideEncryption sse;
+  private ServerSideEncryptionCustomerKey ssec;
   private long objectSize;
   private Map<String, String> headers;
 
@@ -79,7 +79,7 @@ public ComposeSource(
       Long length,
       Map<String, String> headerMap,
       CopyConditions copyConditions,
-      ServerSideEncryption sse)
+      ServerSideEncryptionCustomerKey ssec)
       throws IllegalArgumentException {
     if (bucketName == null) {
       throw new IllegalArgumentException("Source bucket name cannot be empty");
@@ -111,7 +111,7 @@ public ComposeSource(
       this.headerMap = null;
     }
     this.copyConditions = copyConditions;
-    this.sse = sse;
+    this.ssec = ssec;
   }
 
   /** Constructs header . */
@@ -166,8 +166,8 @@ public void buildHeaders(long objectSize, String etag) throws IllegalArgumentExc
       headers.putAll(copyConditions.getConditions());
     }
 
-    if (sse != null) {
-      headers.putAll(sse.copySourceHeaders());
+    if (ssec != null) {
+      headers.putAll(ssec.copySourceHeaders());
     }
 
     this.objectSize = objectSize;
@@ -194,8 +194,8 @@ public CopyConditions copyConditions() {
     return copyConditions;
   }
 
-  public ServerSideEncryption sse() {
-    return sse;
+  public ServerSideEncryptionCustomerKey ssec() {
+    return ssec;
   }
 
   public Map<String, String> headers() {

api/src/main/java/io/minio/DownloadObjectArgs.java

@@ -0,0 +1,58 @@
+/*
+ * MinIO Java SDK for Amazon S3 Compatible Cloud Storage, (C) 2020 MinIO, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.minio;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+public class DownloadObjectArgs extends SsecObjectArgs {
+  private String fileName;
+
+  public String fileName() {
+    return fileName;
+  }
+
+  public static Builder builder() {
+    return new Builder();
+  }
+
+  public static final class Builder extends SsecObjectArgs.Builder<Builder, DownloadObjectArgs> {
+    public Builder fileName(String fileName) {
+      validateFileName(fileName);
+      operations.add(args -> args.fileName = fileName);
+      return this;
+    }
+
+    private void validateFileName(String fileName) {
+      if (fileName == null) {
+        return;
+      }
+
+      if (fileName.isEmpty()) {
+        throw new IllegalArgumentException("filename should be either null or non-empty");
+      }
+
+      Path filePath = Paths.get(fileName);
+      boolean fileExists = Files.exists(filePath);
+
+      if (fileExists && !Files.isRegularFile(filePath)) {
+        throw new IllegalArgumentException(fileName + ": not a regular file");
+      }
+    }
+  }
+}

api/src/main/java/io/minio/GetObjectArgs.java

@@ -0,0 +1,60 @@
+/*
+ * MinIO Java SDK for Amazon S3 Compatible Cloud Storage, (C) 2020 MinIO, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.minio;
+
+public class GetObjectArgs extends SsecObjectArgs {
+  private Long offset;
+  private Long length;
+
+  public Long length() {
+    return length;
+  }
+
+  public Long offset() {
+    return offset;
+  }
+
+  public static Builder builder() {
+    return new Builder();
+  }
+
+  public static final class Builder extends SsecObjectArgs.Builder<Builder, GetObjectArgs> {
+    public Builder offset(Long offset) {
+      validateOffset(offset);
+      operations.add(args -> args.offset = offset);
+      return this;
+    }
+
+    public Builder length(Long length) {
+      validateLength(length);
+      operations.add(args -> args.length = length);
+      return this;
+    }
+
+    private void validateLength(Long length) {
+      if (length != null && length <= 0) {
+        throw new IllegalArgumentException("length should be greater than zero");
+      }
+    }
+
+    private void validateOffset(Long offset) {
+      if (offset != null && offset < 0) {
+        throw new IllegalArgumentException("offset should be zero or greater");
+      }
+    }
+  }
+}