來源：https://blog.csdn.net/qq_40378034/article/details/117851207
現象
最近同事發現新服務用的驅動都是 MySQL8.0，而一些老系統 MySQL 驅動版本較低，存在一些危險漏洞，於是主動的把一些老應用的 MySQL 驅動程序升級到了 8.0。
然而升級後，在併發量較高時，查看監控打點，Druid 連接池拿到連接並執行 SQL 的時間大部分都超過 200ms。
本文詳細的分析了這次“破案”的全過程。
對系統進行壓測，發現出現大量線程阻塞的情況，線程 dump 信息如下：
"http-nio-5366-exec-48" #210 daemon prio=5 os_prio=0 tid=0x00000000023d0800 nid=0x3be9 waiting for monitor entry [0x00007fa4c1400000]
java.lang.Thread.State: BLOCKED (on object monitor)
at org.springframework.boot.web.embedded.tomcat.TomcatEmbeddedWebappClassLoader.loadClass(TomcatEmbeddedWebappClassLoader.java:66)
- waiting to lock <0x0000000775af0960> (a java.lang.Object)
at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1186)
at com.alibaba.druid.util.Utils.loadClass(Utils.java:220)
at com.alibaba.druid.util.MySqlUtils.getLastPacketReceivedTimeMs(MySqlUtils.java:372)
通過上面的信息得知，有線程 BLOCKED 了，BLOCKED 的位置在 com.alibaba.druid.util.Utils.loadClass(Utils.java:220)，於是重點的查看這一塊的代碼，最終發現了問題。
根因分析
public class MySqlUtils {
public static long getLastPacketReceivedTimeMs(Connection conn) throws SQLException {
if (class_connectionImpl == null && !class_connectionImpl_Error) {
try {
class_connectionImpl = Utils.loadClass("com.mysql.jdbc.MySQLConnection");
} catch (Throwable error){
class_connectionImpl_Error = true;
}
}
if (class_connectionImpl == null) {
return -1;
}
if (method_getIO == null && !method_getIO_error) {
try {
method_getIO = class_connectionImpl.getMethod("getIO");
} catch (Throwable error){
method_getIO_error = true;
}
}
if (method_getIO == null) {
return -1;
}
if (class_MysqlIO == null && !class_MysqlIO_Error) {
try {
class_MysqlIO = Utils.loadClass("com.mysql.jdbc.MysqlIO");
} catch (Throwable error){
class_MysqlIO_Error = true;
}
}
if (class_MysqlIO == null) {
return -1;
}
if (method_getLastPacketReceivedTimeMs == null && !method_getLastPacketReceivedTimeMs_error) {
try {
Method method = class_MysqlIO.getDeclaredMethod("getLastPacketReceivedTimeMs");
method.setAccessible(true);
method_getLastPacketReceivedTimeMs = method;
} catch (Throwable error){
method_getLastPacketReceivedTimeMs_error = true;
}
}
if (method_getLastPacketReceivedTimeMs == null) {
return -1;
}
try {
Object connImpl = conn.unwrap(class_connectionImpl);
if (connImpl == null) {
return -1;
}
Object mysqlio = method_getIO.invoke(connImpl);
Long ms = (Long) method_getLastPacketReceivedTimeMs.invoke(mysqlio);
return ms.longValue();
} catch (IllegalArgumentException e) {
throw new SQLException("getLastPacketReceivedTimeMs error", e);
} catch (IllegalAccessException e) {
throw new SQLException("getLastPacketReceivedTimeMs error", e);
} catch (InvocationTargetException e) {
throw new SQLException("getLastPacketReceivedTimeMs error", e);
}
}
MySqlUtils 中的getLastPacketReceivedTimeMs()方法會加載com.mysql.jdbc.MySQLConnection這個類，但在 MySQL 驅動 8.0 中類名改爲com.mysql.cj.jdbc.ConnectionImpl，所以 MySQL 驅動8.0 中加載不到com.mysql.jdbc.MySQLConnection
getLastPacketReceivedTimeMs()方法實現中，如果Utils.loadClass("com.mysql.jdbc.MySQLConnection")加載不到類並拋出異常，會修改變量 class_connectionImpl_Error，下次調用不會再進行加載
public class Utils {
public static Class<?> loadClass(String className) {
Class<?> clazz = null;
if (className == null) {
return null;
}
try {
return Class.forName(className);
} catch (ClassNotFoundException e) {
// skip
}
ClassLoader ctxClassLoader = Thread.currentThread().getContextClassLoader();
if (ctxClassLoader != null) {
try {
clazz = ctxClassLoader.loadClass(className);
} catch (ClassNotFoundException e) {
// skip
}
}
return clazz;
}
但是，在 Utils 的loadClass()方法中同樣 catch了ClassNotFoundException，這就導致loadClass()在加載不到類的時候，並不會拋出異常，從而會導致每調用一次getLastPacketReceivedTimeMs()方法，就會加載一次 MySQLConnection 這個類
線程 dump 信息中可以看到是在調用 TomcatEmbeddedWebappClassLoader 的loadClass()方法時，導致線程阻塞的。
public class TomcatEmbeddedWebappClassLoader extends ParallelWebappClassLoader {
public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
synchronized (JreCompat.isGraalAvailable() ? this : getClassLoadingLock(name)) {
Class<?> result = findExistingLoadedClass(name);
result = (result != null) ? result : doLoadClass(name);
if (result == null) {
throw new ClassNotFoundException(name);
}
return resolveIfNecessary(result, resolve);
}
}
這是因爲 TomcatEmbeddedWebappClassLoader 在加載類的時候，會加synchronized鎖，這就導致每調用一次getLastPacketReceivedTimeMs()方法，就會加載一次com.mysql.jdbc.MySQLConnection，而又始終加載不到，在加載類的時候會加 synchronized 鎖，所以會出現線程阻塞，性能下降的現象。
getLastPacketReceivedTimeMs()方法調用時機
public abstract class DruidAbstractDataSource extends WrapperAdapter implements DruidAbstractDataSourceMBean, DataSource, DataSourceProxy, Serializable {
protected boolean testConnectionInternal(DruidConnectionHolder holder, Connection conn) {
String sqlFile = JdbcSqlStat.getContextSqlFile();
String sqlName = JdbcSqlStat.getContextSqlName();
if (sqlFile != null) {
JdbcSqlStat.setContextSqlFile(null);
}
if (sqlName != null) {
JdbcSqlStat.setContextSqlName(null);
}
try {
if (validConnectionChecker != null) {
boolean valid = validConnectionChecker.isValidConnection(conn, validationQuery, validationQueryTimeout);
long currentTimeMillis = System.currentTimeMillis();
if (holder != null) {
holder.lastValidTimeMillis = currentTimeMillis;
holder.lastExecTimeMillis = currentTimeMillis;
}
if (valid && isMySql) { // unexcepted branch
long lastPacketReceivedTimeMs = MySqlUtils.getLastPacketReceivedTimeMs(conn);
if (lastPacketReceivedTimeMs > 0) {
long mysqlIdleMillis = currentTimeMillis - lastPacketReceivedTimeMs;
if (lastPacketReceivedTimeMs > 0 //
&& mysqlIdleMillis >= timeBetweenEvictionRunsMillis) {
discardConnection(holder);
String errorMsg = "discard long time none received connection. "
+ ", jdbcUrl : " + jdbcUrl
+ ", jdbcUrl : " + jdbcUrl
+ ", lastPacketReceivedIdleMillis : " + mysqlIdleMillis;
LOG.error(errorMsg);
return false;
}
}
}
if (valid && onFatalError) {
lock.lock();
try {
if (onFatalError) {
onFatalError = false;
}
} finally {
lock.unlock();
}
}
return valid;
}
if (conn.isClosed()) {
return false;
}
if (null == validationQuery) {
return true;
}
Statement stmt = null;
ResultSet rset = null;
try {
stmt = conn.createStatement();
if (getValidationQueryTimeout() > 0) {
stmt.setQueryTimeout(validationQueryTimeout);
}
rset = stmt.executeQuery(validationQuery);
if (!rset.next()) {
return false;
}
} finally {
JdbcUtils.close(rset);
JdbcUtils.close(stmt);
}
if (onFatalError) {
lock.lock();
try {
if (onFatalError) {
onFatalError = false;
}
} finally {
lock.unlock();
}
}
return true;
} catch (Throwable ex) {
// skip
return false;
} finally {
if (sqlFile != null) {
JdbcSqlStat.setContextSqlFile(sqlFile);
}
if (sqlName != null) {
JdbcSqlStat.setContextSqlName(sqlName);
}
}
}
只有DruidAbstractDataSource的testConnectionInternal()方法中會調用getLastPacketReceivedTimeMs()方法
testConnectionInternal()是用來檢測連接是否有效的，在獲取連接和歸還連接時都有可能會調用該方法，這取決於Druid檢測連接是否有效的參數
「Druid檢測連接是否有效的參數」：
- testOnBorrow：每次獲取連接時執行validationQuery檢測連接是否有效（會影響性能）
- testOnReturn：每次歸還連接時執行validationQuery檢測連接是否有效（會影響性能）
- testWhileIdle：申請連接的時候檢測，如果空閒時間大於timeBetweenEvictionRunsMillis，執行validationQuery檢測連接是否有效
應用中設置了testOnBorrow=true，每次獲取連接時，都會去搶佔synchronized鎖，所以性能下降的很明顯
解決方案
經驗證，使用Druid 1.x版本<=1.1.22會出現該bug，解決方案就是升級至Druid 1.x版本>=1.1.23或者Druid 1.2.x版本
GitHub issue：https://github.com/alibaba/druid/issues/3808
近期熱文推薦：
1.1,000+ 道 Java面試題及答案整理(2021最新版)
2.別在再滿屏的 if/ else 了，試試策略模式，真香！！
3.臥槽！Java 中的 xx ≠ null 是什麼新語法？
4.Spring Boot 2.5 重磅發佈，黑暗模式太炸了！
覺得不錯，別忘了隨手點贊+轉發哦！