关键词：Slf4j、log4j、可变参数、源码、懒加载、正则

想在打印日志后重复使用要输出的日志结果，所以想封装一下Slf4j可变参数打印的方法，同时拿到打印结果字符串；

本来思路很简单，将可变参数依次填入占位符即可，但是想看看Slf4j是怎么实现的，中间遇到了些问题，记录一下。

1、简单实现

暂时没有想到好方法通过输入的日志级别来动态调用log.info log.error等方法，所以写了三种不同级别的方法；

  FILL_STRING用来判断是否还有未填充的占位符；FILL_FORMAT用来做String.replaceFirst的入参，表示正则表达式匹配"{}"的;

    private static final String FILL_STRING = "{}";
    private static final String FILL_FORMAT = "\\{\\}";


    public static void info(String format, Object... argument) {
        log.info(format, argument);
        String logInfo = getLog(format, argument);
        System.out.println("log is " + logInfo);
    }
    public static void error(String format, Object... argument) {
        log.error(format, argument);
        String logInfo = getLog(format, argument);
        System.out.println("log is " + logInfo);
    }

    public static void warn(String format, Object... argument) {
        log.warn(format, argument);
        String logInfo = getLog(format, argument);
        System.out.println("log is " + logInfo);
    }

    private static String getLog(String format, Object[] arg) {
        // 防止空指针
        if (arg == null) {
            if (!format.contains(FILL_STRING)) {
                return format;
            }
            return format.replaceFirst(FILL_FORMAT, "null");
        }
        StringBuilder formatBuilder = new StringBuilder(format);
        for (Object anArg : arg) {
            // 打印异常堆栈信息
            if (anArg instanceof Throwable) {
                Throwable temp = (Throwable) anArg;
                formatBuilder.append(temp.toString()).append("\n");
                StackTraceElement[] stackTrace = temp.getStackTrace();
                for (StackTraceElement aStackTrace : stackTrace) {
                    formatBuilder.append(aStackTrace).append("\n");
                }
                continue;
            }
            formatBuilder = new StringBuilder(formatBuilder.toString().replaceFirst(FILL_FORMAT, anArg == null ? "null" : anArg.toString()));
        }
        format = formatBuilder.toString();
        return format;
    }

2、查看Slf4j源码

跟踪调用链，跟到org.apache.logging.log4j.core.impl.Log4jLogEvent#Log4jLogEvent构造方法，第412行 

this.message = message;

这句话执行前，message对象的formattedMessage还是null，执行后值就变成填充后的数据了；

刚开始还不能理解，一个赋值操作为什么右边的对象值会改变，后来才知道因为message对象是懒加载，在赋值的时候调用getFormattedMessage方法，如果值为null就进行填充可变参数操作。

附上源码：

    public String getFormattedMessage() {
        if (formattedMessage == null) {
            final StringBuilder buffer = getThreadLocalStringBuilder();
            // 这里填充可变参数
            formatTo(buffer);
            formattedMessage = buffer.toString();
            StringBuilders.trimToMaxSize(buffer, Constants.MAX_REUSABLE_MESSAGE_SIZE);
        }
        return formattedMessage;
    }


    @Override
    public void formatTo(final StringBuilder buffer) {
        if (formattedMessage != null) {
            buffer.append(formattedMessage);
        } else {
            if (indices[0] < 0) {
                ParameterFormatter.formatMessage(buffer, messagePattern, argArray, usedCount);
            } else {
                ParameterFormatter.formatMessage2(buffer, messagePattern, argArray, usedCount, indices);
            }
        }
    }

    /**
     * Replace placeholders in the given messagePattern with arguments.
     *
     * @param buffer the buffer to write the formatted message into
     * @param messagePattern the message pattern containing placeholders.
     * @param arguments      the arguments to be used to replace placeholders.
     */
    static void formatMessage2(final StringBuilder buffer, final String messagePattern,
            final Object[] arguments, final int argCount, final int[] indices) {
        if (messagePattern == null || arguments == null || argCount == 0) {
            buffer.append(messagePattern);
            return;
        }
        int previous = 0;
        for (int i = 0; i < argCount; i++) {
            buffer.append(messagePattern, previous, indices[i]);
            previous = indices[i] + 2;
            recursiveDeepToString(arguments[i], buffer, null);
        }
        buffer.append(messagePattern, previous, messagePattern.length());
    }