It was only after printing out the stack trace of my application that it was hung because the oracle driver was waiting for a secure random number being generated:
Thread[com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread-#2,5,main]
java.io.FileInputStream.readBytes(Native Method)
java.io.FileInputStream.read(FileInputStream.java:255)
sun.security.provider.SeedGenerator$URLSeedGenerator.getSeedBytes(SeedGenerator.java:539)
sun.security.provider.SeedGenerator.generateSeed(SeedGenerator.java:144)
sun.security.provider.SecureRandom$SeederHolder.(SecureRandom.java:203)
sun.security.provider.SecureRandom.engineNextBytes(SecureRandom.java:221)
java.security.SecureRandom.nextBytes(SecureRandom.java:468)
oracle.security.o5logon.O5Logon.a(Unknown Source)
oracle.security.o5logon.O5Logon.(Unknown Source)
oracle.jdbc.driver.T4CTTIoauthenticate.(T4CTTIoauthenticate.java:582)
oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:401)
oracle.jdbc.driver.PhysicalConnection.(PhysicalConnection.java:553)
oracle.jdbc.driver.T4CConnection.(T4CConnection.java:254)
oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:32)
oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:528)
com.mchange.v2.c3p0.DriverManagerDataSource.getConnection(DriverManagerDataSource.java:134)
com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:182)
com.mchange.v2.c3p0.WrapperConnectionPoolDataSource.getPooledConnection(WrapperConnectionPoolDataSource.java:171)
com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool$1PooledConnectionResourcePoolManager.acquireResource(C3P0PooledConnectionPool.java:137)
com.mchange.v2.resourcepool.BasicResourcePool.doAcquire(BasicResourcePool.java:1014)
com.mchange.v2.resourcepool.BasicResourcePool.access$800(BasicResourcePool.java:32)
com.mchange.v2.resourcepool.BasicResourcePool$AcquireTask.run(BasicResourcePool.java:1810)
com.mchange.v2.async.ThreadPoolAsynchronousRunner$PoolThread.run(ThreadPoolAsynchronousRunner.java:547)
java.security.SecureRandom was waiting for some bytes, strange. I googled. Then I had to learn and understand a bit about entropy noise and could realize, with a little help from my Unix friends, that one of the servers had no installed a package that generates the "noise".
As mentioned in the following post I ran the next commands:
cat /proc/sys/kernel/random/entropy_avail
23
cat /proc/sys/kernel/random/poolsize
4096
So there where too few entropy available to generate a secure random number.
After installing the package
cat /proc/sys/kernel/random/entropy_avail
4096
Afterwards I had no more apparent deadlocks, my application could get connections to the DB.