ToolProvider.getSystemJavaCompiler() Return NULL!【转】

import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.Arrays;
import java.util.List;

import javax.tools.JavaCompiler;
import javax.tools.SimpleJavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;
import javax.tools.JavaCompiler.CompilationTask;

/**
* @author YuFa
*
*/
publicclass CompilerTest { 

publicstaticvoid main(String[] args) throws Exception { 
String source = "public class Main {" +
"public static void main(String[] args) {" +
"System.out.println(\"Hello World!\");" +
"} " +
"}"; 


JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fileManager = compiler.getStandardFileManager(null, null, null); 
StringSourceJavaObject sourceObject = new CompilerTest.StringSourceJavaObject("Main", source); 
List<StringSourceJavaObject> fileObjects = Arrays.asList(sourceObject); 
CompilationTask task = compiler.getTask(null, fileManager, null, null, null, fileObjects); 

boolean result = task.call(); 
if (result) { 
System.out.println("Compile succeeded!"); 
} else {
System.out.println("Compile failed!"); 
}
} 

staticclass StringSourceJavaObject extends SimpleJavaFileObject { 

private String content = null; 
public StringSourceJavaObject(String name, String content) throws URISyntaxException { 
super(URI.create("string:///" + name.replace('.','/') + Kind.SOURCE.extension), Kind.SOURCE); 
this.content = content; 
} 

@Override
public CharSequence getCharContent(boolean ignoreEncodingErrors) throws IOException { 
return content; 
} 
} 
}

注意编译上面代码是需要JRE6的，因为JavaCompiler等类都是在JDK6中提供的。当我运行上面代码的时候，结果却抛出了null pointer exception. 问题出在如下这行代码...

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();

ToolProvider.getSystemJavaCompiler()返回的是NULL，因此在调用compiler.getStandardFileManager(null, null, null)的时候抛出了null pointer exception. 

为啥会这样呢？查看了下ToolProvider的源代码，发现如下这么一段....

 

private static final String[] defaultToolsLocation = { "lib", "tools.jar" };privatestatic Class<?> findClass()
throws MalformedURLException, ClassNotFoundException
{
try {
return enableAsserts(Class.forName(defaultJavaCompilerName, false, null));
} catch (ClassNotFoundException e) {
// ignored, try looking else where
}
File file = new File(System.getProperty("java.home"));
if (file.getName().equalsIgnoreCase("jre"))
file = file.getParentFile();
for (String name : defaultToolsLocation)
file = new File(file, name);
URL[] urls = {file.toURI().toURL()};
ClassLoader cl = URLClassLoader.newInstance(urls);
cl.setPackageAssertionStatus("com.sun.tools.javac", true);
return Class.forName(defaultJavaCompilerName, false, cl);
}

我已经设置了JAVA_HOME环境变量，指向了我的JRE安装目录C:\Java\jre6， 注意查找文件的代码...

 

for (String name : defaultToolsLocation)
file = new File(file, name);

 

也就是说会查找目录C:\Java\jre6\lib\tools.jar 

但是注意的是tools.jar并不在jre中，而是在jdk安装目录下，因此我手动把这个jar文件从jdk目录下拷贝到了jre目录下，再次运行上面的代码就没有问题了！同时注意到在代码工程目录下生成了一个名为Main.class的文件，这个确实说明了动态类Main已经编译成功了！