Add a Precondition

Prerequisites

Implementing Liquibase extensions requires an understanding of Java. You will be creating classes, overriding methods, and working with inheritance hierarchies.

Project Setup

If you have not already created a repository to hold your code, see Your First Extension in the Getting Started guide.

Overview

Liquibase ships with a large number of standard preconditions such as:

• tableExists
• columnExists
• sqlCheck
• etc.

but extensions can provide any functionality desired.

When adding support for a new precondition, the interface you are going to implement is liquibase.precondition.Precondition.

Precondition implementations do not need to be thread-safe. Liquibase will generate a new instance of them each time they are used.

Tip

There is a liquibase.precondition.AbstractPrecondition base class you can use which limits the number of methods you must implement.

API Documentation

A complete description of the API, including what methods must be implemented and how is available on the liquibase.precondition.Precondition API page.

Example Code

package com.example.precondition;

import liquibase.Scope;
import liquibase.changelog.ChangeSet;
import liquibase.changelog.DatabaseChangeLog;
import liquibase.changelog.visitor.ChangeExecListener;
import liquibase.database.Database;
import liquibase.exception.*;
import liquibase.executor.Executor;
import liquibase.executor.ExecutorService;
import liquibase.precondition.AbstractPrecondition;
import liquibase.precondition.ErrorPrecondition;
import liquibase.statement.core.RawSqlStatement;

import java.util.Arrays;

public class PasswordIsSetPrecondition extends AbstractPrecondition {

    private String username;

    @Override
    public String getName() {
        return "passwordIsSet";
    }

    @Override
    public Warnings warn(Database database) {
        return new Warnings();
    }

    @Override
    public ValidationErrors validate(Database database) {
        return new ValidationErrors()
                .checkRequiredField("username", getUsername());
    }

    @Override
    public String getSerializedObjectNamespace() {
        return GENERIC_CHANGELOG_EXTENSION_NAMESPACE;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    @Override
    public void check(Database database, DatabaseChangeLog changeLog, ChangeSet changeSet, ChangeExecListener changeExecListener) throws PreconditionFailedException, PreconditionErrorException {
        try {
            Executor executor = Scope.getCurrentScope().getSingleton(ExecutorService.class).getExecutor("jdbc", database);
            String password = executor.queryForObject(new RawSqlStatement("select password from app_users where username='" + database.escapeStringForDatabase(username) + "'"), String.class);

            if (password == null) {
                throw new PreconditionFailedException("Password not set for " + username, changeLog, this);
            }
        } catch (DatabaseException e) {
            throw new PreconditionErrorException(e.getMessage(), Arrays.asList(new ErrorPrecondition(e, changeLog, this)));
        }
    }
}