Code review comment: If this step is done only if the `id` and `url` are not null, then the initialization of `username` and `password` variables can be done within the `if` condition.

Code under review:
public void execute(BuildContext buildContext) {
        CompilerContext context = buildContext.get(BuildContextField.COMPILER_CONTEXT);
        Manifest manifest = ManifestProcessor.getInstance(context).getManifest();
        List<Library> mavenDependencies = new ArrayList<>();
        if (manifest.getPlatform().getLibraries() == null) {
            return;
        }

        String targetRepo = buildContext.get(BuildContextField.TARGET_DIR).toString() + File.separator
                + "platform-libs";
        MavenResolver resolver = new MavenResolver(targetRepo);

        if (manifest.getPlatform().getRepositories() != null && manifest.getPlatform().getRepositories().size() > 0) {
            for (Repository repository : manifest.getPlatform().getRepositories()) {
                String id = repository.getId();
                String url = repository.getUrl();
                String username = repository.getUser();
                String password = repository.getToken();
                if (id != null && url != null) {
                    if (username != null && password != null) {
                        resolver.addRepository(id, url, username, password);
                        continue;
                    }
                    resolver.addRepository(id, url);
                }
            }
        }

        for (Library library : manifest.getPlatform().getLibraries()) {
            if (library.getPath() == null) {
                mavenDependencies.add(library);
            }
        }

        if (mavenDependencies.size() > 0) {
            buildContext.out().println("Resolving Maven dependencies\n\tDownloading dependencies into " + targetRepo);
            for (Library library : mavenDependencies) {
                try {
                    Dependency dependency = resolver.resolve(library.getGroupId(), library.getArtifactId(),
                            library.getVersion(), false);
                    library.setPath(Utils.getJarPath(targetRepo, dependency));
                } catch (MavenResolverException e) {
                    buildContext.err().print("cannot resolve " + library.getArtifactId());
                }
            }
            buildContext.out().println();
        }
    }

Line commented on: if (username != null && password != null) {

Revised code:
public void execute(BuildContext buildContext) {
        CompilerContext context = buildContext.get(BuildContextField.COMPILER_CONTEXT);
        Manifest manifest = ManifestProcessor.getInstance(context).getManifest();
        List<Library> platformLibs = manifest.getPlatform().getLibraries();
        List<Repository> mavenCustomRepos = manifest.getPlatform().getRepositories();
        List<Library> mavenDependencies = new ArrayList<>();
        if (platformLibs == null) {
            return;
        }

        String targetRepo = buildContext.get(BuildContextField.TARGET_DIR).toString() + File.separator
                + "platform-libs";
        MavenResolver resolver = new MavenResolver(targetRepo);

        if (mavenCustomRepos != null && mavenCustomRepos.size() > 0) {
            for (Repository repository : mavenCustomRepos) {
                String id = repository.getId();
                String url = repository.getUrl();
                if (id == null && url == null) {
                    throw createLauncherException("custom maven repository properties are not specified for " +
                            "given platform repository.");
                }

                String username = repository.getUsername();
                String password = repository.getPassword();
                if (username != null && password != null) {
                    resolver.addRepository(id, url, username, password);
                    continue;
                }
                resolver.addRepository(id, url);
            }
        }

        for (Library library : platformLibs) {
            if (library.getPath() == null) {
                if (library.getArtifactId() == null && library.getGroupId() == null && library.getVersion() == null) {
                    throw createLauncherException("path or maven dependency properties are not specified for " +
                            "given platform library dependency.");
                }

                if (library.getArtifactId() == null || library.getGroupId() == null || library.getVersion() == null) {
                    throw createLauncherException("artifact-id, group-id, and version should be specified to " +
                                    "resolve the maven dependency.");
                }
                mavenDependencies.add(library);
            }
        }

        if (mavenDependencies.size() > 0) {
            buildContext.out().println("Resolving Maven dependencies\n\tDownloading dependencies into " + targetRepo);
            for (Library library : mavenDependencies) {
                try {
                    Dependency dependency = resolver.resolve(library.getGroupId(), library.getArtifactId(),
                            library.getVersion(), false);
                    library.setPath(Utils.getJarPath(targetRepo, dependency));
                } catch (MavenResolverException e) {
                    throw createLauncherException("cannot resolve " + library.getArtifactId() + ": " + e.getMessage());
                }
            }
            buildContext.out().println();
        }
    }