Task failed with an exception.
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* What went wrong:
Execution failed for task ':flutter_webview_plugin:compileDebugJavaWithJavac'.
> Could not find tools.jar. Please check that /usr/lib/jvm/java-8-openjdk-amd64 contains a valid JDK installation.
Ubuntu 20.04安裝的open jdk 8,只是JRE,缺少信息 （/usr/lib/jvm/java-8-openjdk-amd64）
需要重新安裝下
sudo apt-get install openjdk-8-jdk
Ubuntu 20.04 上默認使用的是：java-11-openjdk-amd64