反编译还原总结

java class 利用jad 反编译之后，偶尔回碰到一些不正常的代码，例如：label0 :_L1 MISSING_BLOCK_LABEL_30、JVM INSTR ret 7、JVM INSTR tableswitch 1 3: default 269、 JVM INSTR monitorexit、JVM INSTR monitorenter，这些一般是由特殊的for循环、try catch finally语句块、synchronized语句反编译后产生的。下面，就简单介绍一下，一些反编译后的特殊代码的还原规则。

异常  
 
下面的代码前提是类中有如下属性， 
Calendar cal = Calendar.getInstance();
 
1、Exceptioin的还原  
 
反编译后的代码如下： public boolean f1() { return cal.getTime().after(new Date());
Exception e;
e;
e.printStackTrace();
return false;
} 还原后的Java代码 public boolean f1() { try { return cal.getTime().after(new Date());
} catch (Exception e) { e.printStackTrace();
return false;
} } 

2、finally代码的还原 反编译后的Java代码如下： public boolean f2() { boolean flag = cal.getTime().after(new Date());
System.out.println("finally");
return flag;
Exception e;
e;
e.printStackTrace();
System.out.println("finally");
return false;
Exception exception;
exception;
System.out.println("finally");
throw exception;
} 还原后的代码如下： public boolean f2() { try { return cal.getTime().after(new Date());
} catch (Exception e) { e.printStackTrace();
return false;
} finally { System.out.println("finally");
} } 

3、MISSING_BLOCK_LABEL_的还原反编译后的代码 public Object f22() { Date date = cal.getTime();
System.out.println("finally");
return date;
Exception e;
e;
e.printStackTrace();
System.out.println("finally");
break MISSING_BLOCK_LABEL_45;
Exception exception;
exception;
System.out.println("finally");
throw exception;
return null;
} 还原后的Java代码 public Object f22() { try { return cal.getTime();
} catch (Exception e) { e.printStackTrace();
} finally { System.out.println("finally");
} return null;
} 

4、异常中:label的还原反编译后的代码 public String f4() throws Exception { label0: { try { Integer i = new Integer(1);
if(i.intValue() >
0) { System.out.println(i);
break label0;
} System.err.println(i);
} catch(Exception dae) { System.err.println(dae);
throw new RuntimeException(dae);
} return null;
} return "Hello";
} 注意，这个代码有点诡异，实际代码如下： public String f4() throws Exception { try { Integer i = new Integer(1);
if (i.intValue() >
0) { System.out.println(i);
} else { System.err.println(i);
return null;
} return "Hello";
} catch (Exception dae) { System.err.println(dae);
throw new RuntimeException(dae);
} } 

5、典型数据库操作代码还原反编译后代码 public HashMap f5() { Connection conn = null;
HashMap hashmap;
HashMap map = new HashMap();
Class.forName("");
conn = DriverManager.getConnection("jdbc:odbc:");
PreparedStatement pstmt = conn.prepareStatement("select * from table");
pstmt.setString(1, "param");
String columnVallue;
for(ResultSet rs = pstmt.executeQuery();
rs.next();
map.put(columnVallue, "")) columnVallue = rs.getString("column");
hashmap = map;
if(conn != null) try { conn.close();
} catch(SQLException sqlce) { sqlce.printStackTrace();
} return hashmap;
ClassNotFoundException cnfe;
cnfe;
cnfe.printStackTrace();
if(conn != null) try { conn.close();
} catch(SQLException sqlce) { sqlce.printStackTrace();
} break MISSING_BLOCK_LABEL_188;
SQLException sqle;
sqle;
sqle.printStackTrace();
if(conn != null) try { conn.close();
} catch(SQLException sqlce) { sqlce.printStackTrace();
} break MISSING_BLOCK_LABEL_188;
Exception exception;
exception;
if(conn != null) try { conn.close();
} catch(SQLException sqlce) { sqlce.printStackTrace();
} throw exception;
return null;
} 实际代码如下： public HashMap f5() { Connection conn = null;
try { HashMap map = new HashMap();
Class.forName("");
conn = DriverManager.getConnection("jdbc:odbc:");
PreparedStatement pstmt = conn.prepareStatement("select * from table");
pstmt.setString(1, "param");
ResultSet rs = pstmt.executeQuery();
while (rs.next()) { String columnVallue = rs.getString("column");
map.put(columnVallue, "");
} return map;
} catch (ClassNotFoundException cnfe) { cnfe.printStackTrace();
} catch (SQLException sqle) { sqle.printStackTrace();
} finally { if (conn != null) { try { conn.close();
} catch (SQLException sqlce) { sqlce.printStackTrace();
} } } return null;
} 

6、两层异常嵌套代码还原反编译后的代码 public int f6() { int i = cal.getTime().compareTo(new Date());
System.out.println("finally");
return i;
Exception e1;
e1;
e1.printStackTrace();
System.out.println("finally");
return -1;
Exception e2;
e2;
e2.printStackTrace();
System.out.println("finally");
return -2;
Exception exception;
exception;
System.out.println("finally");
throw exception;
} 实际代码 public int f6() { try { try { return cal.getTime().compareTo(new Date());
} catch (Exception e1) { e1.printStackTrace();
return -1;
} } catch (Exception e2) { e2.printStackTrace();
return -2;
} finally { System.out.println("finally");
} } 

7、非常诡异的代码反编译后的代码 public int f7() { int i = cal.getTime().compareTo(new Date());
System.out.println("finally");
return i;
Exception e1;
e1;
e1.printStackTrace();
_L2: System.out.println("finally");
return -1;
Exception e2;
e2;
e2.printStackTrace();
if(true) goto _L2;
else goto _L1 _L1: Exception exception;
exception;
System.out.println("finally");
throw exception;
} 原始代码 public int f7() { try { try { return cal.getTime().compareTo(new Date());
} catch (Exception e1) { e1.printStackTrace();
return -1;
} } catch (Exception e2) { e2.printStackTrace();
return -1;
} finally { System.out.println("finally");
} }