日志最佳实践探究

// 错误示例：无论日志是否打印出来，都会执行 toJson 操作，但是 toJson 耗时很高。
log.info("响应结果: {}", toJson(proceed));


// 正确示例
if (log.isInfoEnabled()) {
    log.info("响应结果:{}", toJson(proceed));
}

// 错误实例：手动提前异常消息
log.error("error param: {}, result: {}, exception: {}",
               toJson(param), toJson(result), exception.getMessage());


// 正确实例
log.error("error param: {}, result: {}",
               toJson(paramDto), toJson(result), exception);

// 错误示例：无意义前缀
log.info("##### 昵称: {}", name));


// 正确示例
log.info("昵称: {}", name));

package org.slf4j.helpers;

import java.text.MessageFormat;
import java.util.HashMap;
import java.util.Map;

final public class MessageFormatter {
    static final char DELIM_START = '{';
    static final char DELIM_STOP = '}';
    static final String DELIM_STR = "{}";
    private static final char ESCAPE_CHAR = '\\';


    // 日志参数拼接的最终实现
    final public static FormattingTuple arrayFormat(final String messagePattern, final Object[] argArray, Throwable throwable) {

        if (messagePattern == null) {
            return new FormattingTuple(null, argArray, throwable);
        }

        if (argArray == null) {
            return new FormattingTuple(messagePattern);
        }

        int i = 0;
        int j;
        // use string builder for better multicore performance
        StringBuilder sbuf = new StringBuilder(messagePattern.length() + 50);

        int L;
        for (L = 0; L < argArray.length; L++) {

            j = messagePattern.indexOf(DELIM_STR, i);

            if (j == -1) {
                // no more variables
                if (i == 0) { // this is a simple string
                    return new FormattingTuple(messagePattern, argArray, throwable);
                } else { // add the tail string which contains no variables and return
                    // the result.
                    sbuf.append(messagePattern, i, messagePattern.length());
                    return new FormattingTuple(sbuf.toString(), argArray, throwable);
                }
            } else {
                if (isEscapedDelimeter(messagePattern, j)) {
                    if (!isDoubleEscaped(messagePattern, j)) {
                        L--; // DELIM_START was escaped, thus should not be incremented
                        sbuf.append(messagePattern, i, j - 1);
                        sbuf.append(DELIM_START);
                        i = j + 1;
                    } else {
                        // The escape character preceding the delimiter start is
                        // itself escaped: "abc x:\\{}"
                        // we have to consume one backward slash
                        sbuf.append(messagePattern, i, j - 1);
                        deeplyAppendParameter(sbuf, argArray[L], new HashMap<Object[], Object>());
                        i = j + 2;
                    }
                } else {
                    // normal case
                    sbuf.append(messagePattern, i, j);
                    // 判断需要拼接，这调用工具方法，进行参数拼接
                    deeplyAppendParameter(sbuf, argArray[L], new HashMap<Object[], Object>());
                    i = j + 2;
                }
            }
        }
        // append the characters following the last {} pair.
        sbuf.append(messagePattern, i, messagePattern.length());
        return new FormattingTuple(sbuf.toString(), argArray, throwable);
    }

    // special treatment of array values was suggested by 'lizongbo'
    private static void deeplyAppendParameter(StringBuilder sbuf, Object o, Map<Object[], Object> seenMap) {
        if (o == null) {
            sbuf.append("null");
            return;
        }
        if (!o.getClass().isArray()) {
            // 简单对象直接拼接
            safeObjectAppend(sbuf, o);
        } else {
            // check for primitive array types because they
            // unfortunately cannot be cast to Object[]
            if (o instanceof boolean[]) {
                booleanArrayAppend(sbuf, (boolean[]) o);
            } else if (o instanceof byte[]) {
                byteArrayAppend(sbuf, (byte[]) o);
            } else if (o instanceof char[]) {
                charArrayAppend(sbuf, (char[]) o);
            } else if (o instanceof short[]) {
                shortArrayAppend(sbuf, (short[]) o);
            } else if (o instanceof int[]) {
                intArrayAppend(sbuf, (int[]) o);
            } else if (o instanceof long[]) {
                longArrayAppend(sbuf, (long[]) o);
            } else if (o instanceof float[]) {
                floatArrayAppend(sbuf, (float[]) o);
            } else if (o instanceof double[]) {
                doubleArrayAppend(sbuf, (double[]) o);
            } else {
                objectArrayAppend(sbuf, (Object[]) o, seenMap);
            }
        }
    }

    // 基于 append 来实现日志参数拼接
    private static void safeObjectAppend(StringBuilder sbuf, Object o) {
        try {
            String oAsString = o.toString();
            sbuf.append(oAsString);
        } catch (Throwable t) {
            Util.report("SLF4J: Failed toString() invocation on an object of type [" + o.getClass().getName() + "]", t);
            sbuf.append("[FAILED toString()]");
        }

    }
}

public class TraceMessageConverter extends MessageConverter {
    @Override
    public String convert(ILoggingEvent event) {
        // 获取 span
        long traceId = span.getTraceId();
        long spanId = span.getSpanId();
        String msg = super.convert(event);
        return "traceId=" + traceId + ", spanId=" + spanId + ", " + msg;
    }
}