How to merge-copy a Java bean

Problem: you have one template object and one target object. They are of the same class and is a standard Java bean. You would like to copy and merge all of the properties from the template object to the target object, but only for attributes that is null in the target.

Solution:

  public <M> void merge(M target, M destination) throws Exception {
    BeanInfo beanInfo = Introspector.getBeanInfo(target.getClass());

    // Iterate over all the attributes
    for (PropertyDescriptor descriptor : beanInfo.getPropertyDescriptors()) {

      // Only copy writable attributes
      if (descriptor.getWriteMethod() != null) {
        Object originalValue = descriptor.getReadMethod()
        .invoke(target);

        // Only copy values values where the destination values is null
        if (originalValue == null) {
          Object defaultValue = descriptor.getReadMethod().invoke(
              destination);
          descriptor.getWriteMethod().invoke(target, defaultValue);
        }

      }
    }
  }

Test code:

  @Test
  public void testMerge() throws Exception {
    Person templatePerson = new Person("Template", 20);
    Person targetPerson = new Person("Johan", null);
    merge(targetPerson, templatePerson);

    assertEquals("Johan", targetPerson.getName());
    assertEquals(20, targetPerson.getAge());
  }

  private static class Person {
    private String name;
    private Integer age;

    public Person(String name, Integer age) {
      this.name = name;
      this.age = age;
    }

    public void setName(String name) {
      this.name = name;
    }
    public String getName() {
      return name;
    }
    public void setAge(Integer age) {
      this.age = age;
    }
    public Integer getAge() {
      return age;
    }
  }