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error日志显示不全的问题

程序员文章站 2022-07-01 20:32:04
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问题描述

某天收到生产环境error日志告警(对error.log监控,超过一定大小就会给开发人员发送告警短信)。但是tail查看最新的异常信息只有这些:java.lang.ClassCastException: null

原因分析

JVM中有个参数:OmitStackTraceInFastThrow,字面意思是省略异常栈信息从而快速抛出,那么JVM是如何做到快速抛出的呢?JVM对一些特定的异常类型做了Fast Throw优化,如果检测到在代码里某个位置连续多次抛出同一类型异常的话,C2会决定用Fast Throw方式来抛出异常,而异常Trace即详细的异常栈信息会被清空。这种异常抛出速度非常快,因为不需要在堆里分配内存,也不需要构造完整的异常栈信息。相关的源码的JVM源码的graphKit.cpp文件中,相关源码如下:

//------------------------------builtin_throw----------------------------------
void GraphKit::builtin_throw(Deoptimization::DeoptReason reason, Node* arg) {
  bool must_throw = true;

  ... ...
  // 首先判断条件是否满足
  // If this particular condition has not yet happened at this
  // bytecode, then use the uncommon trap mechanism, and allow for
  // a future recompilation if several traps occur here.
  // If the throw is hot(表示在代码某个位置重复抛出异常), try to use a more complicated inline mechanism
  // which keeps execution inside the compiled code.
  bool treat_throw_as_hot = false;

  if (ProfileTraps) {
    if (too_many_traps(reason)) {
      treat_throw_as_hot = true;
    }
    // (If there is no MDO at all, assume it is early in
    // execution, and that any deopts are part of the
    // startup transient, and don't need to be remembered.)

    // Also, if there is a local exception handler, treat all throws
    // as hot if there has been at least one in this method.
    if (C->trap_count(reason) != 0
        && method()->method_data()->trap_count(reason) != 0
        && has_ex_handler()) {
        treat_throw_as_hot = true;
    }
  }

  // If this throw happens frequently, an uncommon trap might cause
  // a performance pothole.  If there is a local exception handler,
  // and if this particular bytecode appears to be deoptimizing often,
  // let us handle the throw inline, with a preconstructed instance.
  // Note:   If the deopt count has blown up, the uncommon trap
  // runtime is going to flush this nmethod, not matter what.
  // 这里要满足两个条件:1.检测到频繁抛出异常,2. OmitStackTraceInFastThrow为true,或StackTraceInThrowable为false
  if (treat_throw_as_hot
      && (!StackTraceInThrowable || OmitStackTraceInFastThrow)) {
    // If the throw is local, we use a pre-existing instance and
    // punt on the backtrace.  This would lead to a missing backtrace
    // (a repeat of 4292742) if the backtrace object is ever asked
    // for its backtrace.
    // Fixing this remaining case of 4292742 requires some flavor of
    // escape analysis.  Leave that for the future.
    ciInstance* ex_obj = NULL;
    switch (reason) {
    case Deoptimization::Reason_null_check:
      ex_obj = env()->NullPointerException_instance();
      break;
    case Deoptimization::Reason_div0_check:
      ex_obj = env()->ArithmeticException_instance();
      break;
    case Deoptimization::Reason_range_check:
      ex_obj = env()->ArrayIndexOutOfBoundsException_instance();
      break;
    case Deoptimization::Reason_class_check:
      if (java_bc() == Bytecodes::_aastore) {
        ex_obj = env()->ArrayStoreException_instance();
      } else {
        ex_obj = env()->ClassCastException_instance();
      }
      break;
    }
    ... ...
}

说明:OmitStackTraceInFastThrowStackTraceInThrowable都默认为true,所以条件(!StackTraceInThrowable || OmitStackTraceInFastThrow)为true,即JVM默认开启了Fast Throw优化。如果想关闭这个优化,很简单,配置-XX:-OmitStackTraceInFastThrowStackTraceInThrowable保持默认配置-XX:+OmitStackTraceInFastThrow即可。

另外,根据这段源码的switch .. case ..部分可知,JVM只对几个特定类型异常开启了Fast Throw优化,这些异常包括:

  • NullPointerException
  • ArithmeticException
  • ArrayIndexOutOfBoundsException
  • ArrayStoreException
  • ClassCastException
相关标签: java java