persistence.xml

<property name="javax.persistence.schema-generation.database.action" value="create"/>
<property name="javax.persistence.schema-generation.scripts.action" value="create"/>
<property name="javax.persistence.schema-generation.scripts.create-target" value="create.ddl"/>
<property name="javax.persistence.jdbc.driver" value="org.postgresql.Driver"/>
<property name="javax.persistence.jdbc.url" value="jdbc:postgresql://localhost:5432/db"/>

Java class

import javax.persistence.Persistence;

public class CreateSchema {
	public static void main(String[] args) {
		Persistence.generateSchema("JpaPersistenceUnit", null);
	}
}