Hibernate/JPA Create-Drop Known Issue

Often, running Hibernate/JPA in create drop mode:

  <property name="hibernate.hbm2ddl.auto" value="create-drop"/>

generates multiple errors like this:

  Unsuccessful: alter table XXX drop constraint YYY

Thanks to feedback from Mikko Maunu on StackOverflow, the following issue can be explained by the fact that Hibernate does not check whether an object exists when it deletes it. The underlying database often triggers an error.

Since the create-drop mode drops many objects, these errors happen frequently, but can be ignored.

Parse & Create JSON Documents in Java

The Jackson JSON library provides means to process and create JSON files à la DOM and StAX. It is also possible to process plain old java objects (POJO's). The code samples described here are available at GitHub.

DOM

The following code loads a JSON into a tree structure, then extracts contents by retrieving elements manually:

  String json = "{\"Id\":123456," +
    "\"Title\":\"My book title\"," +
    "\"References\":[\"Reference A\",\"Reference B\"]}";

  System.out.println("Source: " + json);

  // Parsing JSON into tree structure
  ObjectMapper om = new ObjectMapper();
  JsonNode retr = om.readTree(json);

  // Retrieving items from the structure
  JsonNode id_node = retr.get("Id");
  System.out.println("Id" + id_node.asInt());

  JsonNode id_title = retr.get("Title");
  System.out.println("Title " + id_title.asText());

  JsonNode id_refs = retr.get("References");
  System.out.print("References ");

  // Retrieving sub-elements
  Iterator<JsonNode> it = id_refs.elements();

  while ( it.hasNext() ) {
    System.out.print(" " + it.next().asText());
  }

  System.out.println(" ");

The above produces the following:

Source: {"Id":123456,"Title":"My book title","References":["Reference A","Reference B"]}
Id 123456
Title My book title
References Reference A Reference B

StAX

The code below performs a round trip, that is, the manual creation of a JSON and manual parsing with tokens:

JsonFactory jf = new JsonFactory();

// Creating in memory representation
ByteArrayOutputStream baos = new ByteArrayOutputStream();
JsonGenerator jg = jf.createJsonGenerator(
  baos, JsonEncoding.UTF8);

jg.writeStartObject();

jg.writeNumberField("Id", 123456);
jg.writeStringField("Title", "My book title");
jg.writeFieldName("References");

jg.writeStartArray();
jg.writeString("Reference A");
jg.writeString("Reference B");
jg.writeEndArray();

jg.writeEndObject();
jg.close();

// Printing JSON
String result = baos.toString("UTF8");
System.out.println(result);

// Parsing JSON
JsonParser jp = jf.createJsonParser(result);

while (jp.nextToken() != JsonToken.END_OBJECT) {
  String token = jp.getCurrentName();
  if ( "Id".equals(token) || "Title".equals(token) ) {
    System.out.print(token + " ");
    jp.nextToken();
    System.out.println(jp.getText());
  } else if ( "References".equals(token) ) {
    System.out.print(token + " ");
    jp.nextToken(); // JsonToken.START_ARRAY
    while (jp.nextToken() != JsonToken.END_ARRAY) {
      System.out.print(jp.getText() + " ");
    }
    System.out.println("");
  }
}

jp.close();

The produced output is:

{"Id":123456,"Title":"My book title","References":["Reference A","Reference B"]}
Id 123456
Title My book title
References Reference A Reference B

POJO

This example uses a plain old Java object (POJO):

public class PojoItem {

  private int id = 123456;
  private String title;
  private List<String> references
    = new ArrayList<String>() {{
        add("Reference A"); add("Reference B"); }
      };

  public int getId() { 
    return id;
  }

  public void setId(int id) {
    this.id = id;
  }

  public String getTitle() {
    return title;
  }

  public void setTitle(String title) {
    this.title = title;
  }

  public List<String> getReferences() {
    return references;
  }

  public void setReferences(List<String> references) {
    this.references = references;
  }

}

The following transform a POJO into an JSON, and then back into a POJO instance:

PojoItem pi = new PojoItem();
pi.setId(123466);
pi.setTitle("My Title");
pi.setReferences(new ArrayList<String>() { {
  add("Reference A"); add("Reference B");
} });

// Creating and printing JSON
ObjectMapper om = new ObjectMapper();
String result = om.writeValueAsString(pi);
System.out.println(result);

// Parsing JSON
PojoItem user = om.readValue(result, PojoItem.class);

System.out.println("id " + user.getId());
System.out.println("title " + user.getTitle());
System.out.print("references ");
for (String s : user.getReferences()) {
  System.out.print(s + " ");
}

System.out.println(" ");

The above code creates a JSON from a plain old Java object. The output is:

{"id":123466,"title":"My Title","references":["Reference A","Reference B"]}
id 123466
title My Title
references Reference A Reference B

Pojo to XML • Pojo to JSON • JAXB to XML • JAXB to JSON • JAXB Crash Course