情景：電腦上之前安裝了jdk1.8，今天又安裝了jdk1.7，配置idea用jdk1.7執行程式碼，配置完畢，點選執行，報錯：

Error:java: System Java Compiler was not found in classpath:
java.lang.ClassNotFoundException: com.sun.tools.javac.api.JavacTool
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Unknown Source)
	at org.jetbrains.jps.builders.impl.java.JavacCompilerTool.createCompiler(JavacCompilerTool.java:70)
	at org.jetbrains.jps.javac.JavacMain.compile(JavacMain.java:66)
	at org.jetbrains.jps.javac.ExternalJavacProcess.compile(ExternalJavacProcess.java:195)
	at org.jetbrains.jps.javac.ExternalJavacProcess.access$400(ExternalJavacProcess.java:47)
	at org.jetbrains.jps.javac.ExternalJavacProcess$CompilationRequestsHandler$1.run(ExternalJavacProcess.java:262)
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
	at java.util.concurrent.FutureTask.run(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)
 

看到ClassNotFoundException以及JavacTool，果斷開啟cmd，執行javac：

又執行java -version：

java -version沒問題，javac報錯，說明環境變數配置沒有問題，極有可能是Java安裝的問題。

百度得知：

安裝jdk過程中，還會彈出一個jre的安裝，這個和jdk不能安裝在同一目錄下，否則會衝突。

果斷重新安裝，問題解決。