• Maven依赖仲裁

    一、影响依赖的因素

    影响依赖的标签都有哪些

    1. 直接引入具体的依赖信息。注意是不在标签内的情况。如果是在内的情况，请参考2号标签。
    2. 只声明但不发生实际引入，作为依赖管理。依赖管理是指真正发生依赖的时候，再去参考依赖管理的数据。
      这样使用dependency的时候，可以缺省version。
      另外 还可以管控所有的间接依赖，即使间接依赖声明了version，也要被覆盖掉。
    3. 声明自己的父亲，Maven的继承哲学跟Java很类似，因为Maven本身也是用Java实现的，满足单继承。
      一旦子pom继承了父pom，那么会把父pom里的 ，等等属性都继承过来的。当然如果在继承的过程中，出现一样的元素，也是子去覆盖父亲，和Java类似。
      继承时，会分类继承。dependencies继承dependencies，dependencyManagement里的依赖管理只能继承dependencyManagement范围内的依赖管理。
      每一个pom文件都会有一个父亲，即使不声明Parent，也会默认有一个父亲。和Java的Object设计哲学类似。后面在源码分析中我们还会提到。
    4. 代表当前自己的项目的一个属性的集合。
      properties仅仅代表属性的声明，一个属性声明了，和他是否被引用并无关系。我完全可以声明一系列不被人使用的属性。

    依赖的作用域都有哪些

    一个依赖在引入的时候，是可以声明这个依赖的作用范围的。比如这个依赖只对本地起作用，比如只对测试起作用等等。作用域一共有compile，provided，system，test，import，runtime 这几个值。

    简单总结一下：

    • compile和runtime会参与最后的打包环节，其余的都不会。compile可以不写。
    • test只会对 src/test目录下的测试代码起作用。
    • provided是指线上已经提供了这个Jar包，打包的时候不需要在考虑他了，一般像serlvet的包很多都是provided。
    • system和provided没什么太大的区别。
    • import只会出现在dependencyManagement标签内的依赖中，是为了解决Maven的单继承。引入了这个作用域的话，maven会把此依赖的所有的dependencyManagement内的元素加载到当前pom中的，但不会引入当前节点。如下图，并不会引入fastjson作为依赖管理的元素，只是会把fastjson文件定义的依赖管理引入进来。
    <dependencyManagement>
<dependencies>
  <dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.24</version>
    <scope>import</scope>
  </dependency>
<dependencies>
<dependencyManagement>

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    二、单个Jar包内的依赖竞争

    Maven仲裁机制原则

    1. 依赖竞争时，越靠近主干的越优先。
    2. 单颗树在依赖在竞争时(dependencies)（注意：不是dependencyManagement里的dependencies）：
      当deep=1，即直接依赖。同级是靠后优先。
      当deep>1，即间接依赖。同级是靠前优先。
      无论直接依赖还是间接依赖，同级都是靠前优先
    3. 单颗树在依赖管理在竞争时(注意：是dependencyManagement里的dependencies)是靠前优先的。
    4. maven里最重要的2个关系，分别是继承关系和依赖关系。我们所有的规律都应该只从这2个关系入手。

    提炼几点通俗易懂的解释如下

    依赖传递原则

    当在Maven项目中引入A的依赖，A的依赖通常又会引入B的jar包，B可能还会引入C的jar包。这样，当你在pom.xml文件中添加了A的依赖，Maven会自动的帮你把所有相关的依赖都添加进来。
    比如，在Spring Boot项中，当引入了spring-boot-starter-web：

    org.springframework.boot
    spring-boot-starter-web

    我们就可以理解为依赖的传递性。即当一个依赖需要另外一个依赖支撑时，Maven会帮我们把相应的依赖依次添加到项目当中。
    这样的好处是，使用起来就非常方便，不用自己挨个去找依赖Jar包了。坏处是会引起Jar包冲突，我们后面会讲到。

    最短路径优先原则

    依赖链路一：主要根据依赖的路径长短来决定引入哪个依赖（两个冲突的依赖）。
    举例说明：
    依赖链路一：A -> X -> Y -> Z(21.0)
    依赖链路二：B -> Q -> Z(20.0)
    项目中同时引入了A和B两个依赖，它们间接都引入了Z依赖，但由于B的依赖链路比较短，因此最终生效的是Z(20.0)版本。这就是最短路径优先原则。

    最先声明优先原则

    如果两个依赖的路径一样，最短路径优先原则是无法进行判断的，此时需要使用最先声明优先原则，也就是说，谁的声明在前则优先选择。
    举例说明：
    依赖链路一：A -> X -> Z(21.0)
    依赖链路二：B -> Q -> Z(20.0)
    A和B最终都依赖Z，此时A的声明（pom中引入的顺序）优先于B，则针对冲突的Z会优先引入Z(21.0)。

    三. springboot合并打包策略

    多棵树构建顺序原则
    这个功能在Maven的源码中是一个叫Reactor（反应堆）实现的。它主要做了一件事情就是决定一个项目中，多个子pom谁先进行build的顺序，这个出厂顺序很重要，在合并打包时，往往决定了最终谁会在多个pom之间胜出的问题。
    Reactor的原则
    多棵树（多个子pom）构建的顺序是按照被依赖方的要在前，依赖方在后的原则。
    项目要保证这里是不能出现循环依赖的。

    Reactor的原则图解
    如下图子pom1 在被子pom2和子pom3同时依赖，所以子pom1最先被构建，子pom3没有人被依赖，所以最后构建。

    springboot打包策略
    springboot打包会打成一个Fatjar,所有的依赖都会放在BOOT-INF/lib/目录下。
    springboot的打包是越靠后的构建pom越优先，是一种覆盖思想。

    springboot对应的打包插件：

    org.springframework.boot
    spring-boot-maven-plugin

    springboot对应的打包插件代码库：
    https://github.com/spring-projects/spring-boot/tree/main/spring-boot-project/spring-boot-tools/spring-boot-maven-plugin

    对应的打包插件知识库：
    https://docs.spring.io/spring-boot/docs/current/maven-plugin/reference/htmlsingle/

    在项目中调试的话需要在dependencies节点中引入：

    org.springframework.boot
    spring-boot-maven-plugin

    测试工程：
    https://github.com/lzc-alioo/springbootmvndemo

    一次完整依赖分析（通过命令mvn dependency:tree -Dverbose可以查看仲裁说明）：

     ~/work/gitstudy/springbootmvndemo  mvn dependency:tree -Dverbose
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Build Order:
[INFO]
[INFO] springbootmvndemo                                                  [pom]
[INFO] module1                                                            [jar]
[INFO] module2                                                            [jar]
[INFO] start                                                              [jar]
[INFO]
[INFO] -------------------< com.example:springbootmvndemo >--------------------
[INFO] Building springbootmvndemo 0.0.1-SNAPSHOT                          [1/4]
[INFO] --------------------------------[ pom ]---------------------------------
[INFO]
[INFO] --- maven-dependency-plugin:3.3.0:tree (default-cli) @ springbootmvndemo ---
[INFO] com.example:springbootmvndemo:pom:0.0.1-SNAPSHOT
[INFO]
[INFO] ------------------------< com.example:module1 >-------------------------
[INFO] Building module1 0.0.1-SNAPSHOT                                    [2/4]
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-dependency-plugin:3.3.0:tree (default-cli) @ module1 ---
[INFO] com.example:module1:jar:0.0.1-SNAPSHOT
[INFO] \- com.alibaba:fastjson:jar:1.2.46:compile
[INFO]    +- com.squareup.retrofit2:retrofit:jar:2.1.0:provided
[INFO]    |  \- (com.squareup.okhttp3:okhttp:jar:4.9.3:provided - version managed from 3.3.0; omitted for duplicate)
[INFO]    +- com.squareup.okhttp3:okhttp:jar:4.9.3:provided (version managed from 3.6.0)
[INFO]    |  +- com.squareup.okio:okio:jar:2.8.0:provided
[INFO]    |  |  +- (org.jetbrains.kotlin:kotlin-stdlib:jar:1.6.21:provided - version managed from 1.4.0; omitted for duplicate)
[INFO]    |  |  \- org.jetbrains.kotlin:kotlin-stdlib-common:jar:1.6.21:provided (version managed from 1.4.0)
[INFO]    |  \- org.jetbrains.kotlin:kotlin-stdlib:jar:1.6.21:provided (version managed from 1.4.10)
[INFO]    |     +- (org.jetbrains.kotlin:kotlin-stdlib-common:jar:1.6.21:provided - version managed from 1.6.21; omitted for duplicate)
[INFO]    |     \- org.jetbrains:annotations:jar:13.0:provided
[INFO]    +- io.javaslang:javaslang:jar:2.0.6:provided
[INFO]    |  \- io.javaslang:javaslang-match:jar:2.0.6:provided
[INFO]    \- org.glassfish.jersey.core:jersey-common:jar:2.35:provided (version managed from 2.23.2)
[INFO]       +- jakarta.ws.rs:jakarta.ws.rs-api:jar:2.1.6:provided (version managed from 2.1.6)
[INFO]       +- jakarta.annotation:jakarta.annotation-api:jar:1.3.5:provided (version managed from 1.3.5)
[INFO]       +- org.osgi:org.osgi.core:jar:6.0.0:provided
[INFO]       +- org.glassfish.hk2.external:jakarta.inject:jar:2.6.1:provided
[INFO]       \- org.glassfish.hk2:osgi-resource-locator:jar:1.0.3:provided
[INFO]          +- org.osgi:osgi.core:jar:6.0.0:provided
[INFO]          \- org.osgi:osgi.cmpn:jar:6.0.0:provided
[INFO]
[INFO] ------------------------< com.example:module2 >-------------------------
[INFO] Building module2 0.0.1-SNAPSHOT                                    [3/4]
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-dependency-plugin:3.3.0:tree (default-cli) @ module2 ---
[INFO] com.example:module2:jar:0.0.1-SNAPSHOT
[INFO] +- com.example:module1:jar:0.0.1-SNAPSHOT:compile
[INFO] |  \- (com.alibaba:fastjson:jar:1.2.46:compile - omitted for conflict with 1.2.56)
[INFO] \- com.alibaba:fastjson:jar:1.2.56:compile
[INFO]    +- com.squareup.retrofit2:retrofit:jar:2.1.0:provided
[INFO]    |  \- (com.squareup.okhttp3:okhttp:jar:4.9.3:provided - version managed from 3.3.0; omitted for duplicate)
[INFO]    +- com.squareup.okhttp3:okhttp:jar:4.9.3:provided (version managed from 3.6.0)
[INFO]    |  +- com.squareup.okio:okio:jar:2.8.0:provided
[INFO]    |  |  +- (org.jetbrains.kotlin:kotlin-stdlib:jar:1.6.21:provided - version managed from 1.4.0; omitted for duplicate)
[INFO]    |  |  \- org.jetbrains.kotlin:kotlin-stdlib-common:jar:1.6.21:provided (version managed from 1.4.0)
[INFO]    |  \- org.jetbrains.kotlin:kotlin-stdlib:jar:1.6.21:provided (version managed from 1.4.10)
[INFO]    |     +- (org.jetbrains.kotlin:kotlin-stdlib-common:jar:1.6.21:provided - version managed from 1.6.21; omitted for duplicate)
[INFO]    |     \- org.jetbrains:annotations:jar:13.0:provided
[INFO]    +- io.javaslang:javaslang:jar:2.0.6:provided
[INFO]    |  \- io.javaslang:javaslang-match:jar:2.0.6:provided
[INFO]    +- org.glassfish.jersey.core:jersey-common:jar:2.35:provided (version managed from 2.23.2)
[INFO]    |  +- jakarta.ws.rs:jakarta.ws.rs-api:jar:2.1.6:provided (version managed from 2.1.6)
[INFO]    |  +- jakarta.annotation:jakarta.annotation-api:jar:1.3.5:provided (version managed from 1.3.5)
[INFO]    |  +- org.osgi:org.osgi.core:jar:6.0.0:provided
[INFO]    |  +- org.glassfish.hk2.external:jakarta.inject:jar:2.6.1:provided
[INFO]    |  \- org.glassfish.hk2:osgi-resource-locator:jar:1.0.3:provided
[INFO]    |     +- org.osgi:osgi.core:jar:6.0.0:provided
[INFO]    |     \- org.osgi:osgi.cmpn:jar:6.0.0:provided
[INFO]    \- joda-time:joda-time:jar:2.10:provided
[INFO]
[INFO] -------------------------< com.example:start >--------------------------
[INFO] Building start 0.0.1-SNAPSHOT                                      [4/4]
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-dependency-plugin:3.3.0:tree (default-cli) @ start ---
[INFO] com.example:start:jar:0.0.1-SNAPSHOT
[INFO] +- com.example:module1:jar:0.0.1-SNAPSHOT:compile
[INFO] |  \- com.alibaba:fastjson:jar:1.2.46:compile
[INFO] |     +- com.squareup.retrofit2:retrofit:jar:2.1.0:provided
[INFO] |     |  \- (com.squareup.okhttp3:okhttp:jar:4.9.3:provided - version managed from 3.3.0; omitted for duplicate)
[INFO] |     +- com.squareup.okhttp3:okhttp:jar:4.9.3:provided (version managed from 3.6.0)
[INFO] |     |  +- com.squareup.okio:okio:jar:2.8.0:provided
[INFO] |     |  |  +- (org.jetbrains.kotlin:kotlin-stdlib:jar:1.6.21:provided - version managed from 1.4.0; omitted for duplicate)
[INFO] |     |  |  \- org.jetbrains.kotlin:kotlin-stdlib-common:jar:1.6.21:provided (version managed from 1.4.0)
[INFO] |     |  \- org.jetbrains.kotlin:kotlin-stdlib:jar:1.6.21:provided (version managed from 1.4.10)
[INFO] |     |     +- (org.jetbrains.kotlin:kotlin-stdlib-common:jar:1.6.21:provided - version managed from 1.6.21; omitted for duplicate)
[INFO] |     |     \- org.jetbrains:annotations:jar:13.0:provided
[INFO] |     +- io.javaslang:javaslang:jar:2.0.6:provided
[INFO] |     |  \- io.javaslang:javaslang-match:jar:2.0.6:provided
[INFO] |     \- org.glassfish.jersey.core:jersey-common:jar:2.35:provided (version managed from 2.23.2)
[INFO] |        +- jakarta.ws.rs:jakarta.ws.rs-api:jar:2.1.6:provided (version managed from 2.1.6)
[INFO] |        +- (jakarta.annotation:jakarta.annotation-api:jar:1.3.5:provided - version managed from 1.3.5; omitted for duplicate)
[INFO] |        +- org.osgi:org.osgi.core:jar:6.0.0:runtime (scope not updated to runtime)
[INFO] |        +- org.glassfish.hk2.external:jakarta.inject:jar:2.6.1:provided
[INFO] |        \- org.glassfish.hk2:osgi-resource-locator:jar:1.0.3:provided
[INFO] |           +- org.osgi:osgi.core:jar:6.0.0:provided
[INFO] |           \- org.osgi:osgi.cmpn:jar:6.0.0:provided
[INFO] +- com.example:module2:jar:0.0.1-SNAPSHOT:compile
[INFO] |  +- (com.example:module1:jar:0.0.1-SNAPSHOT:compile - version managed from 0.0.1-SNAPSHOT; omitted for duplicate)
[INFO] |  \- (com.alibaba:fastjson:jar:1.2.56:compile - omitted for conflict with 1.2.46)
[INFO] +- org.springframework.boot:spring-boot-starter:jar:2.7.1:compile
[INFO] |  +- org.springframework.boot:spring-boot:jar:2.7.1:compile (version managed from 2.7.1; scope not updated to compile)
[INFO] |  |  +- (org.springframework:spring-core:jar:5.3.21:compile - version managed from 5.3.21; omitted for duplicate)
[INFO] |  |  \- org.springframework:spring-context:jar:5.3.21:compile (version managed from 5.3.21)
[INFO] |  |     +- org.springframework:spring-aop:jar:5.3.21:compile (version managed from 5.3.21)
[INFO] |  |     |  +- (org.springframework:spring-beans:jar:5.3.21:compile - version managed from 5.3.21; omitted for duplicate)
[INFO] |  |     |  \- (org.springframework:spring-core:jar:5.3.21:compile - version managed from 5.3.21; omitted for duplicate)
[INFO] |  |     +- org.springframework:spring-beans:jar:5.3.21:compile (version managed from 5.3.21)
[INFO] |  |     |  \- (org.springframework:spring-core:jar:5.3.21:compile - version managed from 5.3.21; omitted for duplicate)
[INFO] |  |     +- (org.springframework:spring-core:jar:5.3.21:compile - version managed from 5.3.21; omitted for duplicate)
[INFO] |  |     \- org.springframework:spring-expression:jar:5.3.21:compile (version managed from 5.3.21)
[INFO] |  |        \- (org.springframework:spring-core:jar:5.3.21:compile - version managed from 5.3.21; omitted for duplicate)
[INFO] |  +- org.springframework.boot:spring-boot-autoconfigure:jar:2.7.1:compile (version managed from 2.7.1; scope not updated to compile)
[INFO] |  |  \- (org.springframework.boot:spring-boot:jar:2.7.1:compile - version managed from 2.7.1; omitted for duplicate)
[INFO] |  +- org.springframework.boot:spring-boot-starter-logging:jar:2.7.1:compile (version managed from 2.7.1)
[INFO] |  |  +- ch.qos.logback:logback-classic:jar:1.2.11:compile (version managed from 1.2.11)
[INFO] |  |  |  +- ch.qos.logback:logback-core:jar:1.2.11:compile (version managed from 1.2.11)
[INFO] |  |  |  \- (org.slf4j:slf4j-api:jar:1.7.36:compile - version managed from 1.7.32; omitted for duplicate)
[INFO] |  |  +- org.apache.logging.log4j:log4j-to-slf4j:jar:2.17.2:compile (version managed from 2.17.2)
[INFO] |  |  |  +- (org.slf4j:slf4j-api:jar:1.7.36:compile - version managed from 1.7.35; omitted for duplicate)
[INFO] |  |  |  \- org.apache.logging.log4j:log4j-api:jar:2.17.2:compile (version managed from 2.17.2)
[INFO] |  |  |     \- (org.osgi:org.osgi.core:jar:4.3.1:provided - omitted for conflict with 6.0.0)
[INFO] |  |  \- org.slf4j:jul-to-slf4j:jar:1.7.36:compile (version managed from 1.7.36)
[INFO] |  |     \- (org.slf4j:slf4j-api:jar:1.7.36:compile - version managed from 1.7.36; omitted for duplicate)
[INFO] |  +- jakarta.annotation:jakarta.annotation-api:jar:1.3.5:compile (version managed from 1.3.5)
[INFO] |  +- org.springframework:spring-core:jar:5.3.21:compile (version managed from 5.3.21; scope not updated to compile)
[INFO] |  |  \- org.springframework:spring-jcl:jar:5.3.21:compile (version managed from 5.3.21)
[INFO] |  \- org.yaml:snakeyaml:jar:1.30:compile (version managed from 1.30)
[INFO] +- org.springframework.boot:spring-boot-devtools:jar:2.7.1:runtime
[INFO] |  +- (org.springframework.boot:spring-boot:jar:2.7.1:runtime - version managed from 2.7.1; omitted for duplicate)
[INFO] |  \- (org.springframework.boot:spring-boot-autoconfigure:jar:2.7.1:runtime - version managed from 2.7.1; omitted for duplicate)
[INFO] +- org.projectlombok:lombok:jar:1.18.24:compile
[INFO] +- org.springframework.boot:spring-boot-starter-test:jar:2.7.1:test
[INFO] |  +- (org.springframework.boot:spring-boot-starter:jar:2.7.1:test - version managed from 2.7.1; omitted for duplicate)
[INFO] |  +- org.springframework.boot:spring-boot-test:jar:2.7.1:test (version managed from 2.7.1)
[INFO] |  |  \- (org.springframework.boot:spring-boot:jar:2.7.1:test - version managed from 2.7.1; omitted for duplicate)
[INFO] |  +- org.springframework.boot:spring-boot-test-autoconfigure:jar:2.7.1:test (version managed from 2.7.1)
[INFO] |  |  +- (org.springframework.boot:spring-boot:jar:2.7.1:test - version managed from 2.7.1; omitted for duplicate)
[INFO] |  |  +- (org.springframework.boot:spring-boot-test:jar:2.7.1:test - version managed from 2.7.1; omitted for duplicate)
[INFO] |  |  \- (org.springframework.boot:spring-boot-autoconfigure:jar:2.7.1:test - version managed from 2.7.1; omitted for duplicate)
[INFO] |  +- com.jayway.jsonpath:json-path:jar:2.7.0:test (version managed from 2.7.0)
[INFO] |  |  +- net.minidev:json-smart:jar:2.4.8:test (version managed from 2.4.7)
[INFO] |  |  |  \- net.minidev:accessors-smart:jar:2.4.8:test
[INFO] |  |  |     \- org.ow2.asm:asm:jar:9.1:test
[INFO] |  |  \- org.slf4j:slf4j-api:jar:1.7.36:compile (version managed from 1.7.33; scope not updated to compile)
[INFO] |  +- jakarta.xml.bind:jakarta.xml.bind-api:jar:2.3.3:test (version managed from 2.3.3)
[INFO] |  |  \- jakarta.activation:jakarta.activation-api:jar:1.2.2:test (version managed from 1.2.2)
[INFO] |  +- org.assertj:assertj-core:jar:3.22.0:test (version managed from 3.22.0)
[INFO] |  +- org.hamcrest:hamcrest:jar:2.2:runtime (version managed from 2.2; scope not updated to runtime)
[INFO] |  +- org.junit.jupiter:junit-jupiter:jar:5.8.2:test (version managed from 5.8.2)
[INFO] |  |  +- org.junit.jupiter:junit-jupiter-api:jar:5.8.2:test (version managed from 5.8.2)
[INFO] |  |  |  +- org.opentest4j:opentest4j:jar:1.2.0:test
[INFO] |  |  |  +- org.junit.platform:junit-platform-commons:jar:1.8.2:test (version managed from 1.8.2)
[INFO] |  |  |  |  \- (org.apiguardian:apiguardian-api:jar:1.1.2:test - omitted for duplicate)
[INFO] |  |  |  \- org.apiguardian:apiguardian-api:jar:1.1.2:test
[INFO] |  |  +- org.junit.jupiter:junit-jupiter-params:jar:5.8.2:test (version managed from 5.8.2)
[INFO] |  |  |  +- (org.junit.jupiter:junit-jupiter-api:jar:5.8.2:test - version managed from 5.8.2; omitted for duplicate)
[INFO] |  |  |  \- (org.apiguardian:apiguardian-api:jar:1.1.2:test - omitted for duplicate)
[INFO] |  |  \- org.junit.jupiter:junit-jupiter-engine:jar:5.8.2:test (version managed from 5.8.2)
[INFO] |  |     +- org.junit.platform:junit-platform-engine:jar:1.8.2:test (version managed from 1.8.2)
[INFO] |  |     |  +- (org.opentest4j:opentest4j:jar:1.2.0:test - omitted for duplicate)
[INFO] |  |     |  +- (org.junit.platform:junit-platform-commons:jar:1.8.2:test - version managed from 1.8.2; omitted for duplicate)
[INFO] |  |     |  \- (org.apiguardian:apiguardian-api:jar:1.1.2:test - omitted for duplicate)
[INFO] |  |     +- (org.junit.jupiter:junit-jupiter-api:jar:5.8.2:test - version managed from 5.8.2; omitted for duplicate)
[INFO] |  |     \- (org.apiguardian:apiguardian-api:jar:1.1.2:test - omitted for duplicate)
[INFO] |  +- org.mockito:mockito-core:jar:4.5.1:test (version managed from 4.5.1)
[INFO] |  |  +- net.bytebuddy:byte-buddy:jar:1.12.11:test (version managed from 1.12.9)
[INFO] |  |  |  +- net.java.dev.jna:jna:jar:5.8.0:runtime
[INFO] |  |  |  +- (net.java.dev.jna:jna-platform:jar:5.8.0:test - omitted for conflict with 5.7.0)
[INFO] |  |  |  +- com.google.code.findbugs:findbugs-annotations:jar:3.0.1:test
[INFO] |  |  |  \- com.google.code.findbugs:jsr305:jar:3.0.2:runtime (scope not updated to runtime)
[INFO] |  |  +- net.bytebuddy:byte-buddy-agent:jar:1.12.11:test (version managed from 1.12.9)
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[INFO] |  |  |  +- (com.google.code.findbugs:findbugs-annotations:jar:3.0.1:test - omitted for duplicate)
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[INFO] |  |  \- org.objenesis:objenesis:jar:3.2:test
[INFO] |  +- org.mockito:mockito-junit-jupiter:jar:4.5.1:test (version managed from 4.5.1)
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[INFO] |  +- org.skyscreamer:jsonassert:jar:1.5.0:test (version managed from 1.5.0)
[INFO] |  |  \- com.vaadin.external.google:android-json:jar:0.0.20131108.vaadin1:test
[INFO] |  +- (org.springframework:spring-core:jar:5.3.21:test - version managed from 5.3.21; omitted for duplicate)
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[INFO] |  \- org.xmlunit:xmlunit-core:jar:2.9.0:test (version managed from 2.9.0)
[INFO] \- org.springframework.boot:spring-boot-maven-plugin:jar:2.7.1:compile
[INFO]    +- org.springframework.boot:spring-boot-buildpack-platform:jar:2.7.1:runtime (version managed from 2.7.1)
[INFO]    |  +- com.fasterxml.jackson.core:jackson-databind:jar:2.13.3:runtime (version managed from 2.13.3)
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[INFO]    |  |  \- com.fasterxml.jackson.core:jackson-core:jar:2.13.3:runtime (version managed from 2.13.3)
[INFO]    |  +- com.fasterxml.jackson.module:jackson-module-parameter-names:jar:2.13.3:runtime (version managed from 2.13.3)
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[INFO]    |  |  \- (net.java.dev.jna:jna:jar:5.7.0:runtime - omitted for conflict with 5.8.0)
[INFO]    |  +- org.apache.commons:commons-compress:jar:1.21:runtime
[INFO]    |  |  \- (org.osgi:org.osgi.core:jar:6.0.0:runtime - omitted for duplicate)
[INFO]    |  +- org.apache.httpcomponents:httpclient:jar:4.5.13:runtime (version managed from 4.5.13)
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[INFO]    |  |  \- commons-codec:commons-codec:jar:1.15:runtime (version managed from 1.11)
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[INFO]    |  \- org.tomlj:tomlj:jar:1.0.0:runtime
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[INFO]    |     \- (com.google.code.findbugs:jsr305:jar:3.0.2:runtime - omitted for duplicate)
[INFO]    +- org.springframework.boot:spring-boot-loader-tools:jar:2.7.1:runtime (version managed from 2.7.1)
[INFO]    |  +- (org.apache.commons:commons-compress:jar:1.21:runtime - omitted for duplicate)
[INFO]    |  \- (org.springframework:spring-core:jar:5.3.21:runtime - version managed from 5.3.21; omitted for duplicate)
[INFO]    \- org.sonatype.plexus:plexus-build-api:jar:0.0.7:runtime
[INFO]       +- org.codehaus.plexus:plexus-utils:jar:1.5.8:runtime
[INFO]       |  \- org.codehaus.plexus:plexus-interpolation:jar:1.3:runtime
[INFO]       \- org.codehaus.plexus:plexus-container-default:jar:1.0-alpha-9:runtime
[INFO]          +- junit:junit:jar:4.13.2:runtime (version managed from 3.8.1)
[INFO]          |  \- org.hamcrest:hamcrest-core:jar:2.2:runtime (version managed from 1.3)
[INFO]          |     \- (org.hamcrest:hamcrest:jar:2.2:runtime - version managed from 2.2; omitted for duplicate)
[INFO]          +- (org.codehaus.plexus:plexus-utils:jar:1.0.4:runtime - omitted for conflict with 1.5.8)
[INFO]          \- classworlds:classworlds:jar:1.1-alpha-2:runtime
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary for springbootmvndemo 0.0.1-SNAPSHOT:
[INFO]
[INFO] springbootmvndemo .................................. SUCCESS [  0.941 s]
[INFO] module1 ............................................ SUCCESS [  0.106 s]
[INFO] module2 ............................................ SUCCESS [  0.043 s]
[INFO] start .............................................. SUCCESS [  0.272 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  1.876 s
[INFO] Finished at: 2022-07-05T23:18:13+08:00
[INFO] ------------------------------------------------------------------------

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    结论：
    经过分析springboog打包依旧遵循「二、单个Jar包内的依赖竞争」这一段落提到的3个原则：
    依赖传导原则 、最短路径优先原则、最先声明优先原则。

    不同子pom进行构建时会发现引入fastjson版本并不一致，而最终springboot项目打好的fat包中只有一个fastjson版本，那只是因为我们只关心了含main方法的启动类所在module生成的target文件（这个module通常命名是xxx-start，是可以直接运行的程序），也就是说最终生成的fat jar，我们只是使用了这个命名为xxx-start的module进行生成的而已。

    参考文章：
    https://blog.csdn.net/ttzommed/article/details/114984341

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  • 原文地址：https://blog.csdn.net/hl_java/article/details/125630381