fix java apache client optional body, add tests (#14227)

modules/openapi-generator/src/main/resources/Java/libraries/apache-httpclient/ApiClient.mustache

@@ -1059,6 +1059,9 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
       } else {
         throw new ApiException("method " + method + " does not support a request body");
       }
+    } else {
+      // for empty body
+      builder.setEntity(serialize(null, null, contentTypeObj));
     }
 
     try (CloseableHttpResponse response = httpClient.execute(builder.build(), context)) {

samples/client/echo_api/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java

@@ -932,6 +932,9 @@ public class ApiClient extends JavaTimeFormatter {
       } else {
         throw new ApiException("method " + method + " does not support a request body");
       }
+    } else {
+      // for empty body
+      builder.setEntity(serialize(null, null, contentTypeObj));
     }
 
     try (CloseableHttpResponse response = httpClient.execute(builder.build(), context)) {

samples/client/echo_api/java/apache-httpclient/src/test/java/org/openapitools/client/CustomTest.java

@@ -15,10 +15,8 @@ package org.openapitools.client;
 
 import org.junit.Assert;
 import org.openapitools.client.ApiException;
-import org.openapitools.client.api.QueryApi;
+import org.openapitools.client.api.*;
-import org.openapitools.client.model.Category;
+import org.openapitools.client.model.*;
-import org.openapitools.client.model.Pet;
-import org.openapitools.client.model.TestQueryStyleFormExplodeTrueArrayStringQueryObjectParameter;
 import org.junit.Test;
 import org.junit.Ignore;
 
@@ -31,7 +29,29 @@ import java.util.*;
 public class CustomTest {
 
     private final QueryApi api = new QueryApi();
+    private final BodyApi bodyApi = new BodyApi();
 
+    /**
+     * Test body parameter(s)
+     * <p>
+     * Test body parameter(s)
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void testEchoBodyPet() throws ApiException {
+        Pet queryObject = new Pet().id(12345L).name("Hello World").
+                photoUrls(Arrays.asList(new String[]{"http://a.com", "http://b.com"})).category(new Category().id(987L).name("new category"));
+
+        Pet p = bodyApi.testEchoBodyPet(queryObject);
+        Assert.assertNotNull(p);
+        Assert.assertEquals("Hello World", p.getName());
+        Assert.assertEquals(Long.valueOf(12345L), p.getId());
+
+        // response is empty body
+        Pet p2 = bodyApi.testEchoBodyPet(null);
+        Assert.assertNull(p2);
+    }
 
     /**
      * Test query parameter(s)

samples/client/petstore/java/apache-httpclient/src/main/java/org/openapitools/client/ApiClient.java

@@ -1075,6 +1075,9 @@ public class ApiClient extends JavaTimeFormatter {
       } else {
         throw new ApiException("method " + method + " does not support a request body");
       }
+    } else {
+      // for empty body
+      builder.setEntity(serialize(null, null, contentTypeObj));
     }
 
     try (CloseableHttpResponse response = httpClient.execute(builder.build(), context)) {