OnExpressionCondition Class — spring-boot Architecture

Source Code

core/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/OnExpressionCondition.java lines 40–84

@Order(Ordered.LOWEST_PRECEDENCE - 20)
class OnExpressionCondition extends SpringBootCondition {

	@Override
	public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
		Map<String, @Nullable Object> attributes = metadata
			.getAnnotationAttributes(ConditionalOnExpression.class.getName());
		Assert.state(attributes != null, "'attributes' must not be null");
		String expression = (String) attributes.get("value");
		Assert.state(expression != null, "'expression' must not be null");
		expression = wrapIfNecessary(expression);
		ConditionMessage.Builder messageBuilder = ConditionMessage.forCondition(ConditionalOnExpression.class,
				"(" + expression + ")");
		expression = context.getEnvironment().resolvePlaceholders(expression);
		ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
		if (beanFactory != null) {
			boolean result = evaluateExpression(beanFactory, expression);
			return new ConditionOutcome(result, messageBuilder.resultedIn(result));
		}
		return ConditionOutcome.noMatch(messageBuilder.because("no BeanFactory available."));
	}

	private boolean evaluateExpression(ConfigurableListableBeanFactory beanFactory, String expression) {
		BeanExpressionResolver resolver = beanFactory.getBeanExpressionResolver();
		if (resolver == null) {
			resolver = new StandardBeanExpressionResolver();
		}
		BeanExpressionContext expressionContext = new BeanExpressionContext(beanFactory, null);
		Object result = resolver.evaluate(expression, expressionContext);
		return (result != null && (boolean) result);
	}

	/**
	 * Allow user to provide bare expression with no '#{}' wrapper.
	 * @param expression source expression
	 * @return wrapped expression
	 */
	private String wrapIfNecessary(String expression) {
		if (!expression.startsWith("#{")) {
			return "#{" + expression + "}";
		}
		return expression;
	}

}