參考文章：
malloc/free和new/delete必須成對(duì)出現(xiàn)，以防止內(nèi)存泄露
一、什么時(shí)候垃圾回收：
簡單說：
當(dāng)一塊內(nèi)存被jvm通過它自己的認(rèn)證機(jī)制認(rèn)為不再被調(diào)用的時(shí)候才會(huì)在
它認(rèn)為合適的時(shí)機(jī)進(jìn)行回收；
具體說：
The job of the garbage collector is to find objects that are no longer needed by an application and to remove them when they can no longer be accessed or referenced. The garbage collector starts at the root nodes, classes that persist throughout the life of a Java application, and sweeps through all of the nodes that are referenced. As it traverses the nodes, it keeps track of which objects are actively being referenced. Any classes that are no longer being referenced are then eligible to be garbage collected. The memory resources used by these objects can be returned to the Java virtual machine (JVM) when the objects are deleted.
So it is true that Java code does not require the programmer to be responsible for memory management cleanup, and that it automatically garbage collects unused objects. However, the key point to remember is that an object is only counted as being unused when it is no longer referenced.
垃圾收集器的工作是找到由一個(gè)應(yīng)用程序不再需要的對(duì)象，在他們不再被訪問或引用將其刪除。垃圾收集器從根節(jié)點(diǎn)、在整個(gè)Java應(yīng)用的生命中存在的類開始，并通過掃描所有被引用的節(jié)點(diǎn)。由于它遍歷的節(jié)點(diǎn)，它跟蹤哪些對(duì)象正在積極引用。任何不再被引用的的類，然后才有資格被垃圾收集。當(dāng)對(duì)象被刪除時(shí)，他們所占用的內(nèi)存資源，才被Java虛擬機(jī)（JVM）回收。
二、什么樣的java代碼容易memory leak?
1.首先一種情況是collection或者是map一直被put數(shù)據(jù)，沒有機(jī)會(huì)remove，導(dǎo)致OutOfMemoryError。尤其是當(dāng)collection或者是map被設(shè)計(jì)成static變量的時(shí)候，它就是個(gè)global性質(zhì)的變量，很可能永遠(yuǎn)不會(huì)被賦為null。這也是不建議使用static變量的一個(gè)原因。
2.
在listener的模式下，如果listener一直在注冊(cè)register而沒有機(jī)會(huì)remove也會(huì)導(dǎo)致OutOfMemoryError。其實(shí)listener也是一個(gè)list的結(jié)構(gòu)，本質(zhì)上是一樣的。很多l(xiāng)istener是以匿名類被構(gòu)造和注冊(cè)到被監(jiān)聽類上面去的， 而被監(jiān)聽類如果也沒有正確remove注冊(cè)的listener的話也會(huì)導(dǎo)致OutOfMemoryError。
待續(xù)...........
訪問登錄jsp時(shí)：java.lang.LinkageError: loader constraints violated when linking javax/el/ExpressionFactory class；
參考文章：
http://www.cnblogs.com/ztf2008/archive/2009/03/17/1413965.html解釋：
加載時(shí)違背約束條件。
錯(cuò)誤的原因:
tomcat/lib下的el-api.jar與項(xiàng)目WEB-INF/lib目錄下的el-api.jar沖突。
解決方式：
把項(xiàng)目目錄下的el-api.jar刪除即可。
本人總結(jié)原因：項(xiàng)目應(yīng)部署在tomcat5.5上，部署在tomcat6.0上會(huì)報(bào)上面的jar包沖突的錯(cuò)誤
導(dǎo)入數(shù)據(jù)的時(shí)候，MYSQL 報(bào)錯(cuò):Data too long for column
解決辦法:
在my.ini里找到(此文件在mysql安裝目錄下)
sql-mode=”STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION”
把其中的STRICT_TRANS_TABLES,去掉,
或者把sql-mode=STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
注釋掉，然后重啟mysql就ok了 ！