How to re-index an index in Elasticsearch using Java ?

To re-index an index using java, build a re-index request using ReindexRequestBuilder API like:

ReindexRequestBuilder reindexRequest = 
new ReindexRequestBuilder(client,ReindexAction.INSTANCE)
    .source("source_index")
    .destination("destination_index")
    .refresh(true);

After creating a request execute the request:

reindexRequest.execute();

To validate whether the request is executed or not add a validation check:

if(copy.execute().isDone()) {
System.out.println("Request is executed");
}

Bingo! Your index is re-indexed.

Reflections in java

Reflection is a powerful feature of Java which provides the ability to inspect & modify the code at run time (manipulate internal properties of the program).

For example: It’s possible for a Java class to obtain the names of all its members and display them. Even we can also use reflection to instantiate an object, invoke it’s methods and change field values.

 

How it is done?

For every object JVM creates an immutable Class object which is used by reflection to get the run time properties of that object and once it has access we can change the properties. Reflection is not something which is used in daily programming tasks as it has some cons as well, one being a security threat, as using reflection we can get access to the private variables of a class and then can change it’s value.

 

How do we get access to the class object?

object.getClass();

 

After having the access we can get the methods, variables and constructors etc.

 

Stop the world phase

Garbage Collection literally stops the world.

When a GC occurs in young generation space, it is completed quickly as the young generation space is small.

Young generation space is the space where newly instantiated objects are stored. Internally, this space has two survivor spaces which are used when GC occurs and the objects which still have references are shifted to a survivor space. If an object survives many cycles of GC, it is shifted to old generation space.

Problem is when GC occurs in Old generation space which contains long lived objects. This space uses a lot more memory than the young generation and when GC occurs in old generation, it literally halts all the requests made to that JVM process.

So, the world literally stops !!

Why Java 8 ?

In simple words java 8 allows us to write code more precisely and concisely, which is better than writing verbose code in the java versions prior to java 8.

Example: Let’s sort a collection of cars based on their speed.

Java versions prior to java 8 :

Collections.sort(fleet, new Comparator() {
  @Override
  public int compare (Car c1, Car c2) { 
  return c1.getSpeed().compareTo(c2.getSpeed());
  }
}

Instead of writing a verbose code like above, using java 8 we can write the same code as:

Java 8 :

fleet.sort(Comparator.comparing(Car::getSpeed));

The above code is more concise and could be read as “sort fleet comparing Car’s speed”.

So why write a boilerplate code which is not related to the problem statement. Instead you can write concise code which is related to the problem statement and has SQL like readability.