不同项目开发时，其基础的组件很可能是一样的，例如springboot,数据库连接池，驱动等等。如果把这些相同的JAR在tomcat中共享，这样既可以节约内存，也可以减少
各应用的大小。
我用的是Tomcat9，各版本差别不大。
具体做法：
1.catalina.properties
蓝色部分就是需要修改的部分
# Licensed to the Apache Software Foundation (ASF) under one or more
# contributor license agreements. See the NOTICE file distributed with
# this work for additional information regarding copyright ownership.
# The ASF licenses this file to You under the Apache License, Version 2.0
# (the "License"); you may not use this file except in compliance with
# the License. You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
# List of comma-separated packages that start with or equal this string
# will cause a security exception to be thrown when
# passed to checkPackageAccess unless the
# corresponding RuntimePermission ("accessClassInPackage."+package) has
# been granted.
package.access=sun.,org.apache.catalina.,org.apache.coyote.,org.apache.jasper.,org.apache.tomcat.
#
# List of comma-separated packages that start with or equal this string
# will cause a security exception to be thrown when
# passed to checkPackageDefinition unless the
# corresponding RuntimePermission ("defineClassInPackage."+package) has
# been granted.
#
# by default, no packages are restricted for definition, and none of
# the class loaders supplied with the JDK call checkPackageDefinition.
#
package.definition=sun.,java.,org.apache.catalina.,org.apache.coyote.,
org.apache.jasper.,org.apache.naming.,org.apache.tomcat.
#
#
# List of comma-separated paths defining the contents of the "common"
# classloader. Prefixes should be used to define what is the repository type.
# Path may be relative to the CATALINA_HOME or CATALINA_BASE path or absolute.
# If left as blank,the JVM system loader will be used as Catalina's "common"
# loader.
# Examples:
# "foo": Add this folder as a class repository
# "foo/*.jar": Add all the JARs of the specified folder as class
# repositories
# "foo/bar.jar": Add bar.jar as a class repository
#
# Note: Values are enclosed in double quotes ("...") in case either the
# ${catalina.base} path or the ${catalina.home} path contains a comma.
# Because double quotes are used for quoting, the double quote character
# may not appear in a path.
common.loader="${catalina.base}/lib","${catalina.base}/lib/*.jar","${catalina.home}/lib","${catalina.home}/lib/*.jar","${catalina.base}/shared/lib","${catalina.base}/shared/lib/*.jar"
#
# List of comma-separated paths defining the contents of the "server"
# classloader. Prefixes should be used to define what is the repository type.
# Path may be relative to the CATALINA_HOME or CATALINA_BASE path or absolute.
# If left as blank, the "common" loader will be used as Catalina's "server"
# loader.
# Examples:
# "foo": Add this folder as a class repository
# "foo/*.jar": Add all the JARs of the specified folder as class
# repositories
# "foo/bar.jar": Add bar.jar as a class repository
#
# Note: Values may be enclosed in double quotes ("...") in case either the
# ${catalina.base} path or the ${catalina.home} path contains a comma.
# Because double quotes are used for quoting, the double quote character
# may not appear in a path.
server.loader=
#
# List of comma-separated paths defining the contents of the "shared"
# classloader. Prefixes should be used to define what is the repository type.
# Path may be relative to the CATALINA_BASE path or absolute. If left as blank,
# the "common" loader will be used as Catalina's "shared" loader.
# Examples:
# "foo": Add this folder as a class repository
# "foo/*.jar": Add all the JARs of the specified folder as class
# repositories
# "foo/bar.jar": Add bar.jar as a class repository
# Please note that for single jars, e.g. bar.jar, you need the URL form
# starting with file:.
#
# Note: Values may be enclosed in double quotes ("...") in case either the
# ${catalina.base} path or the ${catalina.home} path contains a comma.
# Because double quotes are used for quoting, the double quote character
# may not appear in a path.
shared.loader="${catalina.base}/shared/lib","${catalina.base}/shared/lib/*.jar"
# Default list of JAR files that should not be scanned using the JarScanner
# functionality. This is typically used to scan JARs for configuration
# information. JARs that do not contain such information may be excluded from
# the scan to speed up the scanning process. This is the default list. JARs on
# this list are excluded from all scans. The list must be a comma separated list
# of JAR file names.
# The list of JARs to skip may be over-ridden at a Context level for individual
# scan types by configuring a JarScanner with a nested JarScanFilter.
# The JARs listed below include:
# - Tomcat Bootstrap JARs
# - Tomcat API JARs
# - Catalina JARs
# - Jasper JARs
# - Tomcat JARs
# - Common non-Tomcat JARs
# - Test JARs (JUnit, Cobertura and dependencies)
tomcat.util.scan.StandardJarScanFilter.jarsToSkip=
annotations-api.jar,
ant-junit*.jar,
ant-launcher.jar,
ant.jar,
asm-*.jar,
aspectj*.jar,
bootstrap.jar,
catalina-ant.jar,
catalina-ha.jar,
catalina-ssi.jar,
catalina-storeconfig.jar,
catalina-tribes.jar,
catalina.jar,
cglib-*.jar,
cobertura-*.jar,
commons-beanutils*.jar,
commons-codec*.jar,
commons-collections*.jar,
commons-daemon.jar,
commons-dbcp*.jar,
commons-digester*.jar,
commons-fileupload*.jar,
commons-httpclient*.jar,
commons-io*.jar,
commons-lang*.jar,
commons-logging*.jar,
commons-math*.jar,
commons-pool*.jar,
dom4j-*.jar,
easymock-*.jar,
ecj-*.jar,
el-api.jar,
geronimo-spec-jaxrpc*.jar,
h2*.jar,
hamcrest-*.jar,
hibernate*.jar,
httpclient*.jar,
icu4j-*.jar,
jasper-el.jar,
jasper.jar,
jaspic-api.jar,
jaxb-*.jar,
jaxen-*.jar,
jdom-*.jar,
jetty-*.jar,
jmx-tools.jar,
jmx.jar,
jsp-api.jar,
jstl.jar,
jta*.jar,
junit-*.jar,
junit.jar,
log4j*.jar,
mail*.jar,
objenesis-*.jar,
oraclepki.jar,
oro-*.jar,
servlet-api-*.jar,
servlet-api.jar,
slf4j*.jar,
taglibs-standard-spec-*.jar,
tagsoup-*.jar,
tomcat-api.jar,
tomcat-coyote.jar,
tomcat-dbcp.jar,
tomcat-i18n-*.jar,
tomcat-jdbc.jar,
tomcat-jni.jar,
tomcat-juli-adapters.jar,
tomcat-juli.jar,
tomcat-util-scan.jar,
tomcat-util.jar,
tomcat-websocket.jar,
tools.jar,
websocket-api.jar,
wsdl4j*.jar,
xercesImpl.jar,
xml-apis.jar,
xmlParserAPIs-*.jar,
xmlParserAPIs.jar,
xom-*.jar
# Default list of JAR files that should be scanned that overrides the default
# jarsToSkip list above. This is typically used to include a specific JAR that
# has been excluded by a broad file name pattern in the jarsToSkip list.
# The list of JARs to scan may be over-ridden at a Context level for individual
# scan types by configuring a JarScanner with a nested JarScanFilter.
tomcat.util.scan.StandardJarScanFilter.jarsToScan=
log4j-taglib*.jar,
log4j-web*.jar,
log4javascript*.jar,
slf4j-taglib*.jar
# String cache configuration.
tomcat.util.buf.StringCache.byte.enabled=true
#tomcat.util.buf.StringCache.char.enabled=true
#tomcat.util.buf.StringCache.trainThreshold=500000
#tomcat.util.buf.StringCache.cacheSize=50002.建立shared/lib 这两个目录
3.把共享的jar放入shared/lib中.
注意：共享jar虽然带来的性能的提升，但是也会引发其他问题，例如redis缓存读取时发生类型转换错误，只有一个项目访问redis是没有问题的，其它的不行。还有logback日志输出，也只有一个项目生成日志。