jni函数signature

关键字： JNI, GetMethodID, java.lang.NoSuchMethodError, javap, 内部类

20180816 tjy

转载请注明出处

这几天维护一个app， 需要把一些事情放到so文件中去实现。
在写jni调用java方法的时候，遇到方法的signature错误导致GetMethodID调用抛出java.lang.NoSuchMethodError错误，在解决这个问题上面花了一些时间。
最后各种百度，发现javap能很好的解决这个问题。

javap -s的解释如下，即打印signature. 这个 signature正是写JNI函数需要的。

-s Print internal type signatures

john@john-ThinkPad-Edge-E435:~/workspace/temp$ javap --help
Usage: javap  
where possible options include:
  -help  --help  -?        Print this usage message
  -version                 Version information
  -v  -verbose             Print additional information
  -l                       Print line number and local variable tables
  -public                  Show only public classes and members
  -protected               Show protected/public classes and members
  -package                 Show package/protected/public classes
                           and members (default)
  -p  -private             Show all classes and members
  -c                       Disassemble the code
  -s                       Print internal type signatures
  -sysinfo                 Show system info (path, size, date, MD5 hash)
                           of class being processed
  -constants               Show static final constants
  -classpath         Specify where to find user class files
  -bootclasspath     Override location of bootstrap class files

具体JNI的使用方法就不在这里展开了，直接上代码和运行结果，希望能给遇到类似问题的人一个思路。

package com.test;

class Test {
    private int mX;
    private InternalClass mIc;

    public void setX(int x) {
        mX = x;
    }
    public int getX() {
        return mX;
    }

    public void setIc(InternalClass ic) {
        mIc = ic;
    }

    public InternalClass getIc() {
        return mIc;
    }

    //internal class
    public class InternalClass  {
        private String mStr;

        public String toString() {
            return mStr;
        }
    }

    //main
    public static void main() {
        Test test = new Test();
    }
}

编译之后得到如下的结果:

john@john-ThinkPad-Edge-E435:~/workspace/temp$ javac Test.java 
john@john-ThinkPad-Edge-E435:~/workspace/temp$ 
john@john-ThinkPad-Edge-E435:~/workspace/temp$ ls
Test.class  Test$InternalClass.class  Test.java  Test.java~

javap -s的运行结果如下：
其中有一个setIc函数的参数是个内部类。可以看下内部类的写法。

john@john-ThinkPad-Edge-E435:~/workspace/temp$ javap -s Test.class 
Compiled from "Test.java"
class com.test.Test {
  com.test.Test();
    Signature: ()V

  public void setX(int);
    Signature: (I)V

  public int getX();
    Signature: ()I

  public void setIc(com.test.Test$InternalClass);
    Signature: (Lcom/test/Test$InternalClass;)V

  public com.test.Test$InternalClass getIc();
    Signature: ()Lcom/test/Test$InternalClass;

  public static void main();
    Signature: ()V
}
john@john-ThinkPad-Edge-E435:~/workspace/temp$ 

在看内部类的javap -s的结果:

john@john-ThinkPad-Edge-E435:~/workspace/temp$ javap -s Test\$InternalClass.class 
Compiled from "Test.java"
public class com.test.Test$InternalClass {
  final com.test.Test this$0;
    Signature: Lcom/test/Test;
  public com.test.Test$InternalClass(com.test.Test);
    Signature: (Lcom/test/Test;)V

  public java.lang.String toString();
    Signature: ()Ljava/lang/String;
}
john@john-ThinkPad-Edge-E435:~/workspace/temp$