logback default file appender location

We have a J2EE web application with logback as the logging function. We use Tomcat 7 as the server. We are using the logback ConsoleAppender right now, so I can go to /var/log/tomcat7/catalina.out to check the log.

Right now, we want to move the log into a file. I will use a RollingFileAppender to save the log.

I DO NOT want to use a absolute path like:

<appender name="FILE" class="xxx.RollingFileAppender">
    <file>/var/log/testFile1.log</file>
    <encoder>
      <pattern>%msg%n</pattern>
    </encoder>
</appender>

I DO NOT want to use a system environment variable, either, like:

<appender name="FILE" class="xxx.RollingFileAppender">
    <file>{LOG_LOCATION}\testFile2.log</file>
    <encoder>
      <pattern>%msg%n</pattern>
    </encoder>
</appender>

If I only write the name of log file in the tag, like:

<appender name="FILE" class="xxx.RollingFileAppender">
    <file>testFile3.log</file>
    <encoder>
      <pattern>%msg%n</pattern>
    </encoder>
</appender>

I will receive the error:

21:29:45,265 |-ERROR in ch.qos.logback.core.FileAppender[TEST_LOG] - openFile(testFile3.log,true) call failed. java.io.FileNotFoundException: testFile3.log (Permission denied)
        at java.io.FileNotFoundException: testFile3.log (Permission denied)
        at      at java.io.FileOutputStream.open(Native Method)
        at      at java.io.FileOutputStream.<init>(FileOutputStream.java:221)
        at      at ch.qos.logback.core.recovery.ResilientFileOutputStream.<init>(ResilientFileOutputStream.java:28)
        at      at ch.qos.logback.core.FileAppender.openFile(FileAppender.java:149)
        at      at ch.qos.logback.core.FileAppender.start(FileAppender.java:108)
        at      at ch.qos.logback.core.joran.action.AppenderAction.end(AppenderAction.java:96)
        at      at ch.qos.logback.core.joran.spi.Interpreter.callEndAction(Interpreter.java:317)
        at      at ch.qos.logback.core.joran.spi.Interpreter.endElement(Interpreter.java:196)
        at      at ch.qos.logback.core.joran.spi.Interpreter.endElement(Interpreter.java:182)
        at      at ch.qos.logback.core.joran.spi.EventPlayer.play(EventPlayer.java:62)
        at      at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:149)
        at      at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:135)
        at      at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:99)
        at      at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:49)
        at      at ch.qos.logback.classic.util.ContextInitializer.configureByResource(ContextInitializer.java:75)
        at      at ch.qos.logback.classic.util.ContextInitializer.autoConfig(ContextInitializer.java:148)
        at      at org.slf4j.impl.StaticLoggerBinder.init(StaticLoggerBinder.java:85)
        at      at org.slf4j.impl.StaticLoggerBinder.<clinit>(StaticLoggerBinder.java:55)
        at      at org.slf4j.LoggerFactory.bind(LoggerFactory.java:128)
        at      at org.slf4j.LoggerFactory.performInitialization(LoggerFactory.java:107)
        at      at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:295)
        at      at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:269)
        at      at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:156)
        at      at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:132)
        at      at org.apache.commons.logging.LogFactory.getLog(LogFactory.java:657)
        at      at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:282)
        at      at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:106)
        at      at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4779)
        at      at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5273)
        at      at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
        at      at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:895)
        at      at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:871)
        at      at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:615)
        at      at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:958)
        at      at org.apache.catalina.startup.HostConfig$DeployWar.run(HostConfig.java:1599)
        at      at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
        at      at java.util.concurrent.FutureTask.run(FutureTask.java:262)
        at      at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at      at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at      at java.lang.Thread.run(Thread.java:744)

My question is whether I can create a log file only with its name or not. And if I can create it, where it the default location for the logback log file in Tomcat 7 server.