编译简单程序时Java编译器失败

Question

我有Java编译器的问题.我简化了我的代码:

package a;

public class Base {
  // compiles if this is made public or an int
  protected Integer value = 0;
}

---

package b; // must be in a separate package

import a.Base;

public class Sub extends Base {
  public void increment() {
    System.out.println(super.value);
    value++;
    super.value = 1;
    super.value = super.value + 1;
    // this line crashes the compiler; the others all work
    super.value++;
  }
}

编译后我得到:

Information:java: An exception has occurred in the compiler (1.8.0_51). Please file a bug at the Java Developer Connection (http://java.sun.com/webapps/bugreport)  after checking the Bug Parade for duplicates. Include your program and the following diagnostic in your report.  Thank you.
Information:java: java.lang.NullPointerException
Information:java:   at com.sun.tools.javac.code.Symbol$ClassSymbol.isSubClass(Symbol.java:1020)
Information:java:   at com.sun.tools.javac.comp.Lower.accessClass(Lower.java:1108)
Information:java:   at com.sun.tools.javac.comp.Lower.accessSymbol(Lower.java:983)
Information:java:   at com.sun.tools.javac.comp.Lower.access(Lower.java:1220)
Information:java:   at com.sun.tools.javac.comp.Lower.visitSelect(Lower.java:3855)
Information:java:   at com.sun.tools.javac.tree.JCTree$JCFieldAccess.accept(JCTree.java:1897)
Information:java:   at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:58)
Information:java:   at com.sun.tools.javac.comp.Lower.translate(Lower.java:2371)
Information:java:   at com.sun.tools.javac.comp.Lower.translate(Lower.java:2382)
Information:java:   at com.sun.tools.javac.comp.Lower.visitVarDef(Lower.java:3547)
Information:java:   at com.sun.tools.javac.tree.JCTree$JCVariableDecl.accept(JCTree.java:852)
Information:java:   at com.sun.tools.javac.tree.TreeTranslator.translateVarDefs(TreeTranslator.java:78)
Information:java:   at com.sun.tools.javac.comp.Lower.visitLetExpr(Lower.java:3859)
Information:java:   at com.sun.tools.javac.tree.JCTree$LetExpr.accept(JCTree.java:2426)
Information:java:   at com.sun.tools.javac.comp.Lower.visitLetExpr(Lower.java:3860)
Information:java:   at com.sun.tools.javac.comp.Lower.visitUnary(Lower.java:3319)
Information:java:   at com.sun.tools.javac.tree.JCTree$JCUnary.accept(JCTree.java:1746)
Information:java:   at com.sun.tools.javac.tree.TreeTranslator.visitExec(TreeTranslator.java:245)
Information:java:   at com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1296)
Information:java:   at com.sun.tools.javac.tree.TreeTranslator.translate(TreeTranslator.java:70)
Information:java:   at com.sun.tools.javac.tree.TreeTranslator.visitBlock(TreeTranslator.java:162)
Information:java:   at com.sun.tools.javac.comp.Lower.visitBlock(Lower.java:3561)
Information:java:   at com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:909)
Information:java:   at com.sun.tools.javac.tree.TreeTranslator.visitMethodDef(TreeTranslator.java:145)
Information:java:   at com.sun.tools.javac.comp.Lower.visitMethodDefInternal(Lower.java:2828)
Information:java:   at com.sun.tools.javac.comp.Lower.visitMethodDef(Lower.java:2737)
Information:java:   at com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:778)
Information:java:   at com.sun.tools.javac.comp.Lower.visitClassDef(Lower.java:2508)
Information:java:   at com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:693)
Information:java:   at com.sun.tools.javac.comp.Lower.translate(Lower.java:2390)
Information:java:   at com.sun.tools.javac.comp.Lower.translateTopLevelClass(Lower.java:3932)
Information:java:   at com.sun.tools.javac.main.JavaCompiler.desugar(JavaCompiler.java:1512)
Information:java:   at com.sun.tools.javac.main.JavaCompiler.desugar(JavaCompiler.java:1356)
Information:java:   at com.sun.tools.javac.main.JavaCompiler.compile2(JavaCompiler.java:901)
Information:java:   at com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:860)
Information:java:   at com.sun.tools.javac.main.Main.compile(Main.java:523)
Information:java:   at com.sun.tools.javac.api.JavacTaskImpl.doCall(JavacTaskImpl.java:129)
Information:java:   at com.sun.tools.javac.api.JavacTaskImpl.call(JavacTaskImpl.java:138)
Information:java:   at org.jetbrains.jps.javac.JavacMain.compile(JavacMain.java:168)
Information:java:   at org.jetbrains.jps.incremental.java.JavaBuilder.compileJava(JavaBuilder.java:382)
Information:java:   at org.jetbrains.jps.incremental.java.JavaBuilder.compile(JavaBuilder.java:296)
Information:java:   at org.jetbrains.jps.incremental.java.JavaBuilder.doBuild(JavaBuilder.java:204)
Information:java:   at org.jetbrains.jps.incremental.java.JavaBuilder.build(JavaBuilder.java:176)
Information:java:   at org.jetbrains.jps.incremental.IncProjectBuilder.runModuleLevelBuilders(IncProjectBuilder.java:1202)
Information:java:   at org.jetbrains.jps.incremental.IncProjectBuilder.runBuildersForChunk(IncProjectBuilder.java:877)
Information:java:   at org.jetbrains.jps.incremental.IncProjectBuilder.buildTargetsChunk(IncProjectBuilder.java:948)
Information:java:   at org.jetbrains.jps.incremental.IncProjectBuilder.buildChunkIfAffected(IncProjectBuilder.java:840)
Information:java:   at org.jetbrains.jps.incremental.IncProjectBuilder.buildChunks(IncProjectBuilder.java:665)
Information:java:   at org.jetbrains.jps.incremental.IncProjectBuilder.runBuild(IncProjectBuilder.java:372)
Information:java:   at org.jetbrains.jps.incremental.IncProjectBuilder.build(IncProjectBuilder.java:193)
Information:java:   at org.jetbrains.jps.cmdline.BuildRunner.runBuild(BuildRunner.java:137)
Information:java:   at org.jetbrains.jps.cmdline.BuildSession.runBuild(BuildSession.java:293)
Information:java:   at org.jetbrains.jps.cmdline.BuildSession.run(BuildSession.java:124)
Information:java:   at org.jetbrains.jps.cmdline.BuildMain$MyMessageHandler$1.run(BuildMain.java:242)
Information:java:   at org.jetbrains.jps.service.impl.SharedThreadPoolImpl$1.run(SharedThreadPoolImpl.java:41)
Information:java:   at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
Information:java:   at java.util.concurrent.FutureTask.run(FutureTask.java:266)
Information:java:   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
Information:java:   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
Information:java:   at java.lang.Thread.run(Thread.java:745)
Information:java: Errors occurred while compiling module 'testAGH'
Information:2015-10-23 14:23 - Compilation completed with 1 error and 0 warnings in 671ms
Error:java: Compilation failed: internal java compiler error

为什么Java无法编译？

Answer 1

这显然是一个编译器错误。您应该按照输出指示提交错误，但既然您问为什么我会尝试探索这个:)

JDK langtools 存储库包含javac. 我浏览了几个不同的版本，但无法找到行号与堆栈跟踪匹配的版本（我不确定如何将Java 版本与版本号匹配），所以我要离开最新版本b132。

正如您所看到的，根本问题是NullPointerExceptionin com.sun.tools.javac.code.Symbolin Symbol.ClassSymbol.isSubClass()。这就是问题的根源。在堆栈跟踪的较高位置我们可以看到到达之前的access()调用。因此我们可以推断编译器在失败时正在尝试验证子类是否允许访问。accessSymbol()accessClass()isSubClass()super.value

如果不知道 NPE 发生在哪一行，就很难查明确切的问题，因此不幸的是，这里的情况变得模糊。由于这种失败仅在我们尝试递增并设置（++或+=）字段时发生，并且仅当我们将其引用为 时才会发生super.value，因此我们期望看到一些与强制转换、自动装箱、赋值或算术相关的代码。唉，似乎没有任何isSubClass()事情与这些真正相关。

我的猜测是，考虑到 中似乎没有任何罪证isSubClass()，那就是Lower.accessClass()将 a 传递null到 中isSubClass()。此方法中传递了三个值isSubClass()：字段currentClass（as c，这是“当前封闭的类”），sym.owner（我认为sym是我们的value字段，我不知道它owner是什么，但大概是它在其中声明的类），以及types，它看起来总是被设置为非空值。所以它是currentClassor sym.owner，并且之前有sym.owner对 NPE 的引用Lower.access()，所以它似乎也是非空的。

所以我敢说罪魁祸首Lower.currentClass出于某种原因是无效的。由于currentClass是一个包私有的非最终字段，我不会费心去尝试确定它null此时是否可以，但它似乎是可能的。当然，还有其他几个可能的罪魁祸首isSubClass()，所以我很可能是错的。

尚无定论，但我希望这能提供信息！如果我了解更多，我一定会更新。