Apache Log4j是一个基于Java的日志记录工具，用起来非常方便，但是Log4j 1.x如果使用不慎，会引起死锁问题，进行导致整个网站的宕机
示例如下：
log4j 配置
<?
xml
version
=
"1.0"
encoding
=
"UTF-8"
?>
<!DOCTYPE log4j:configuration PUBLIC "-//APACHE//DTD LOG4J 1.2//EN" "log4j.dtd">
<
appender
name
=
"console"
class
=
"org.apache.log4j.ConsoleAppender"
>
<
param
name
=
"Target"
value
=
"System.out"
/>
<
layout
class
=
"org.apache.log4j.PatternLayout"
>
<
param
name
=
"ConversionPattern"
value
=
"%d %t %-5p (%c:%L) - %m%n"
/>
</
layout
>
</
appender
>
<
logger
name
=
"a"
additivity
=
"false"
>
<
level
value
=
"info"
/>
<
appender-ref
ref
=
"console"
/>
</
logger
>
<
logger
name
=
"b"
additivity
=
"false"
>
<
level
value
=
"info"
/>
<
appender-ref
ref
=
"console"
/>
</
logger
>
<
root
>
<
appender-ref
ref
=
"console"
/>
</
root
>
</
log4j:configuration
>
|
使用logger a进行日志输出的java文件
package
a;
import
org.apache.log4j.Logger;
public
class
A {
private
static
final
Logger logger = Logger.getLogger(A.
class
);
public
A() {
try
{
logger.info(
"[A.A()] class A"
);
}
catch
(Exception ex) {
logger.error(
"[A.A()]"
, ex);
}
}
}
|
使用logger b进行日志输出的java文件
package
b;
import
org.apache.log4j.Logger;
import
a.*;
public
class
B {
private
static
final
Logger logger = Logger.getLogger(B.
class
.getName());
public
static
void
main(String[] args) {
new
Thread(
new
Runnable() {
public
void
run() {
logger.info(
new
B());
}
}).start();
new
A();
}
public
String toString() {
logger.info(
"[B.toString]"
);
new
A();
return
"[B.toString] finish"
;
}
}
|
运行上面的程序，相当大的概率，程序会卡在这里不动，通过jstack命令，我们可以发现程序死锁了
jstack 3876
|
2016-12-22 16:54:38
Full
thread
dump Java HotSpot(TM) 64-Bit Server VM (25.65-b01 mixed mode):
"Attach Listener"
#12 daemon prio=9 os_prio=31 tid=0x00007fb932944000 nid=0x3b0b waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Thread-0"
#11 prio=5 os_prio=31 tid=0x00007fb9341c1800 nid=0x5903 waiting
for
monitor entry [0x000070000175b000]
java.lang.Thread.State: BLOCKED (on object monitor)
at org.apache.log4j.Category.callAppenders(Category.java:205)
- waiting to lock (a org.apache.log4j.Logger)
at org.apache.log4j.Category.forcedLog(Category.java:391)
at org.apache.log4j.Category.info(Category.java:666)
at a.A.(A.java:11)
at b.B.toString(B.java:18)
at org.apache.log4j.or.DefaultRenderer.doRender(DefaultRenderer.java:37)
at org.apache.log4j.or.RendererMap.findAndRender(RendererMap.java:80)
at org.apache.log4j.spi.LoggingEvent.getRenderedMessage(LoggingEvent.java:368)
at org.apache.log4j.helpers.PatternParser$BasicPatternConverter.convert(PatternParser.java:402)
at org.apache.log4j.helpers.PatternConverter.format(PatternConverter.java:65)
at org.apache.log4j.PatternLayout.format(PatternLayout.java:506)
at org.apache.log4j.WriterAppender.subAppend(WriterAppender.java:310)
at org.apache.log4j.WriterAppender.append(WriterAppender.java:162)
at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
- locked (a org.apache.log4j.ConsoleAppender)
at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
at org.apache.log4j.Category.callAppenders(Category.java:206)
- locked (a org.apache.log4j.Logger)
at org.apache.log4j.Category.forcedLog(Category.java:391)
at org.apache.log4j.Category.info(Category.java:666)
at b.B$1.run(B.java:10)
at java.lang.Thread.run(Thread.java:745)
"Monitor Ctrl-Break"
#10 daemon prio=5 os_prio=31 tid=0x00007fb9358de800 nid=0x5703 runnable [0x0000700001658000]
java.lang.Thread.State: RUNNABLE
at java.net.PlainSocketImpl.socketAccept(Native Method)
at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:409)
at java.net.ServerSocket.implAccept(ServerSocket.java:545)
at java.net.ServerSocket.accept(ServerSocket.java:513)
at com.intellij.rt.execution.application.AppMain$1.run(AppMain.java:90)
at java.lang.Thread.run(Thread.java:745)
"Service Thread"
#9 daemon prio=9 os_prio=31 tid=0x00007fb933002800 nid=0x5303 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C1 CompilerThread3"
#8 daemon prio=9 os_prio=31 tid=0x00007fb936020800 nid=0x5103 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread2"
#7 daemon prio=9 os_prio=31 tid=0x00007fb935007800 nid=0x4f03 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread1"
#6 daemon prio=9 os_prio=31 tid=0x00007fb935006800 nid=0x4d03 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread0"
#5 daemon prio=9 os_prio=31 tid=0x00007fb933801000 nid=0x4b03 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Signal Dispatcher"
#4 daemon prio=9 os_prio=31 tid=0x00007fb935837800 nid=0x3a13 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Finalizer"
#3 daemon prio=8 os_prio=31 tid=0x00007fb933802000 nid=0x3803 in Object.wait() [0x0000700000d3a000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.
remove
(ReferenceQueue.java:143)
- locked (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.
remove
(ReferenceQueue.java:164)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)
"Reference Handler"
#2 daemon prio=10 os_prio=31 tid=0x00007fb936009000 nid=0x3603 in Object.wait() [0x0000700000c37000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:502)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:157)
- locked (a java.lang.ref.Reference$Lock)
"main"
#1 prio=5 os_prio=31 tid=0x00007fb935801800 nid=0x1703 waiting
for
monitor entry [0x0000700000218000]
java.lang.Thread.State: BLOCKED (on object monitor)
at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:231)
- waiting to lock (a org.apache.log4j.ConsoleAppender)
at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
at org.apache.log4j.Category.callAppenders(Category.java:206)
- locked (a org.apache.log4j.Logger)
at org.apache.log4j.Category.forcedLog(Category.java:391)
at org.apache.log4j.Category.info(Category.java:666)
at a.A.(A.java:11)
at b.B.main(B.java:13)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
"VM Thread"
os_prio=31 tid=0x00007fb936006800 nid=0x3403 runnable
"GC task thread#0 (ParallelGC)"
os_prio=31 tid=0x00007fb93580a800 nid=0x2403 runnable
"GC task thread#1 (ParallelGC)"
os_prio=31 tid=0x00007fb932804800 nid=0x2603 runnable
"GC task thread#2 (ParallelGC)"
os_prio=31 tid=0x00007fb93580b000 nid=0x2803 runnable
"GC task thread#3 (ParallelGC)"
os_prio=31 tid=0x00007fb936003000 nid=0x2a03 runnable
"GC task thread#4 (ParallelGC)"
os_prio=31 tid=0x00007fb93580b800 nid=0x2c03 runnable
"GC task thread#5 (ParallelGC)"
os_prio=31 tid=0x00007fb936003800 nid=0x2e03 runnable
"GC task thread#6 (ParallelGC)"
os_prio=31 tid=0x00007fb936004000 nid=0x3003 runnable
"GC task thread#7 (ParallelGC)"
os_prio=31 tid=0x00007fb936004800 nid=0x3203 runnable
"VM Periodic Task Thread"
os_prio=31 tid=0x00007fb935838000 nid=0x5503 waiting on condition
JNI global references: 60
Found one Java-level deadlock:
=============================
"Thread-0"
:
waiting to lock monitor 0x00007fb93600edf8 (object 0x000000076b2a21d0, a org.apache.log4j.Logger),
which is held by
"main"
"main"
:
waiting to lock monitor 0x00007fb93600d8a8 (object 0x000000076b311008, a org.apache.log4j.ConsoleAppender),
which is held by
"Thread-0"
Java stack information
for
the threads listed above:
===================================================
"Thread-0"
:
at org.apache.log4j.Category.callAppenders(Category.java:205)
- waiting to lock (a org.apache.log4j.Logger)
at org.apache.log4j.Category.forcedLog(Category.java:391)
at org.apache.log4j.Category.info(Category.java:666)
at a.A.(A.java:11)
at b.B.toString(B.java:18)
at org.apache.log4j.or.DefaultRenderer.doRender(DefaultRenderer.java:37)
at org.apache.log4j.or.RendererMap.findAndRender(RendererMap.java:80)
at org.apache.log4j.spi.LoggingEvent.getRenderedMessage(LoggingEvent.java:368)
at org.apache.log4j.helpers.PatternParser$BasicPatternConverter.convert(PatternParser.java:402)
at org.apache.log4j.helpers.PatternConverter.format(PatternConverter.java:65)
at org.apache.log4j.PatternLayout.format(PatternLayout.java:506)
at org.apache.log4j.WriterAppender.subAppend(WriterAppender.java:310)
at org.apache.log4j.WriterAppender.append(WriterAppender.java:162)
at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
- locked (a org.apache.log4j.ConsoleAppender)
at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
at org.apache.log4j.Category.callAppenders(Category.java:206)
- locked (a org.apache.log4j.Logger)
at org.apache.log4j.Category.forcedLog(Category.java:391)
at org.apache.log4j.Category.info(Category.java:666)
at b.B$1.run(B.java:10)
at java.lang.Thread.run(Thread.java:745)
"main"
:
at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:231)
- waiting to lock (a org.apache.log4j.ConsoleAppender)
at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
at org.apache.log4j.Category.callAppenders(Category.java:206)
- locked (a org.apache.log4j.Logger)
at org.apache.log4j.Category.forcedLog(Category.java:391)
at org.apache.log4j.Category.info(Category.java:666)
at a.A.(A.java:11)
at b.B.main(B.java:13)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
Found 1 deadlock.
|
jstack命令的最后，给我们一个很重要的提示，就是程序发生了死锁，那么接下来，就是要分析程序为什么会产生死锁, jstack命令也给了相应的提示信息，如下所示
"Thread-0"
:
waiting to lock monitor
0x00007fb93600edf8
(object
0x000000076b2a21d0
, a org.apache.log4j.Logger),
which is held by
"main"
"main"
:
waiting to lock monitor
0x00007fb93600d8a8
(object
0x000000076b311008
, a org.apache.log4j.ConsoleAppender),
which is held by
"Thread-0"
|
Thread-0尝试去lock一个Logger, org.apache.log4j.Logger这个类继承于org.apache.log4j.category，而调用的info方法在category这个类中，由org.apache.log4j.category中这段源码可以发现，Thread-0实际上是去尝试lock一个category
public
void
callAppenders(LoggingEvent event) {
int
writes =
0
;
for
(Category c =
this
; c !=
null
; c=c.parent) {
// Protected against simultaneous call to addAppender, removeAppender,...
synchronized
(c) {
if
(c.aai !=
null
) {
writes += c.aai.appendLoopOnAppenders(event);
}
if
(!c.additive) {
break
;
}
}
}
if
(writes ==
0
) {
repository.emitNoAppenderWarning(
this
);
}
}
|
从上面的代码中可以看出Thread-0实际上在等待lock一个category,再结合上面的log4j的配置文件，很容易的发现，Thread-0会使用两个logger文件，即name=a与name=b的logger文件均会使用，当调用logger.info(new B())的时候，实际上最后会被自动的转化为对new B().toString()方法的调用，在这个方法里面即输出了A的日志，又输出了B的日志，再结合jstack命令下面的描述，可以发现再lock [name=a的logger时，由于已经被其他线程占用，因此 Thread-0 block]
"Thread-0":
at org.apache.log4j.Category.callAppenders(Category.java:205)
- waiting to lock (a org.apache.log4j.Logger)
at org.apache.log4j.Category.forcedLog(Category.java:391)
at org.apache.log4j.Category.info(Category.java:666)
at a.A.(A.java:11)
at b.B.toString(B.java:18)
at org.apache.log4j.or.DefaultRenderer.doRender(DefaultRenderer.java:37)
|
综上所述，Thread-0线程拥有了logger name=b的category,ConsoleAppender ,且正在尝试去获取logger name=a的category
jstack命令显示main线程正在wait一个ConsoleAppender,由源码可以发现ConsoleAppender继承于AppenderSkeleton，再结合以下输出
at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:231)
- waiting to lock (a org.apache.log4j.ConsoleAppender)
at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
at org.apache.log4j.Category.callAppenders(Category.java:206)
|
可以发现，main线程在执行AppenderSkeleton.java中doAppend方法时发生了wait,看源码如下所示
public
synchronized
void
doAppend(LoggingEvent event) {
if
(closed) {
LogLog.error(
"Attempted to append to closed appender named ["
+name+
"]."
);
return
;
}
if
(!isAsSevereAsThreshold(event.getLevel())) {
return
;
}
Filter f =
this
.headFilter;
FILTER_LOOP:
while
(f !=
null
) {
switch
(f.decide(event)) {
case
Filter.DENY:
return
;
case
Filter.ACCEPT:
break
FILTER_LOOP;
case
Filter.NEUTRAL: f = f.getNext();
}
}
this
.append(event);
}
|
看到了synchronized,也就明白了，main线程无法获取到ConsoleAppender的对象锁，因为此对象锁正在被Thread-0占用，综上所述，main线程拥有了logger name=a的category,正在尝试去获取ConsoleAppender的对象锁，去进行日志输出，因此两线程死锁了
Log4j的日志输出流程
由上面的流程图可以看出，在一个日志的输出过程中，会尝试进行两次lock,在高并发的情况下，两次lock就很容易造成死锁，上面产生死锁的可能执行序列如下:
1.Thread-0 lock b的category
2.Thread-0 lock ConsoleAppender
3.main线程 lock a的category
4.Thread-0 尝试去获取 a的category,因为要进行日志a的输出了, block
5.main线程尝试去lock ConsoleAppender，由于被Thread-0占用, bloc
最终死锁了!
找到死锁发生的原因，那如何解决这个问题:
取决于log4j配置文件的合理性
在log4j的上面加上slf4j,将要输出的日志提前格式化成一个字符串，对于上面的例子，也就是将logger.info(new A()) 中new A().toString()提前调用，没有slf4j时的调用序列是
lock b->lock consoleappender->lock a->lock consoleappender 是一个步骤完成的
加入slf4j后的设用序列就变成了
lock a -> lock consoleappender 完成了第一阶段new A()
lock b -> lock consoleappender 完成了第二阶段 logger.info(string)
加入slf4j后可以发现的是日志的输出分成两步执行了，因为执行logger.info时，会首先对要输出的内容进行格式化，将其格式化成一个string, so,不会发生死锁了。
转载原文：http://www.aiprograming.com/b/pengpeng/23