1 00:00:01,110 --> 00:00:05,300 So let's go ahead and look at few other important bytecode instructions. 2 00:00:05,500 --> 00:00:12,720 One minute if you're doing water and instructions called in work special on Iraq which we'll just recap 3 00:00:12,720 --> 00:00:19,560 here like in the case of a real CPA you JVM that is an abstract computing machine also has an instruction 4 00:00:19,560 --> 00:00:20,610 set. 5 00:00:20,710 --> 00:00:28,140 We know that a method bytecode stream would act as instructions for the JVM and the bytecode instructions 6 00:00:28,230 --> 00:00:34,440 also have a similar representation as regular instructions office EPU each bytecode instruction basically 7 00:00:34,440 --> 00:00:40,230 involves a run byte opcode followed by 0 or more options. 8 00:00:40,230 --> 00:00:45,750 The awkward specifies the operation to be performed when the operands specific additional information 9 00:00:45,760 --> 00:00:47,010 . 10 00:00:47,010 --> 00:00:52,310 For example the I add instruction that we saw earlier was just were adding two values. 11 00:00:52,740 --> 00:00:59,160 In this case I add is the opcode and there were no options open and so actually implicit. 12 00:00:59,160 --> 00:01:05,430 The upward sort implies popping the end values at the top of the operand stack and performing the necessary 13 00:01:05,820 --> 00:01:07,490 operation. 14 00:01:07,530 --> 00:01:10,050 We also have that Iler instruction. 15 00:01:10,050 --> 00:01:13,780 One example was I load underscore one here the suffix one. 16 00:01:13,800 --> 00:01:21,330 So says the opera and that is the instruction is telling JVM to dig the value at index 1 in the local 17 00:01:21,360 --> 00:01:29,910 variables Ari on push it onto the top of the operands like like C.P. you DBMSs execution engine also 18 00:01:29,910 --> 00:01:31,880 follows fetch on execution lifecycle. 19 00:01:31,950 --> 00:01:38,490 And we know that that is GBM would fetch an upcourt on any trailing operands then it would execute the 20 00:01:38,490 --> 00:01:45,230 operation requested by that upcourt and then it fetches the next instruction and so on. 21 00:01:45,240 --> 00:01:50,490 Now let's use this simple program to review the bytecode instructions that it generates. 22 00:01:50,490 --> 00:01:53,580 You can also find the program in resource section. 23 00:01:53,580 --> 00:01:59,220 The program has a main method and one additional method called string which basically overrides the 24 00:01:59,430 --> 00:02:02,310 string method in the Object class. 25 00:02:02,310 --> 00:02:06,650 This method has a system that overprint Allen's statement underlines the string hello. 26 00:02:07,260 --> 00:02:12,780 The main one that creates an instance of this class and simply in with a string method on that new instance 27 00:02:14,090 --> 00:02:20,370 not to look at the bytecode we will use a jalopy too which is basically a bytecode disassembler that 28 00:02:20,370 --> 00:02:22,210 comes with it. 29 00:02:22,290 --> 00:02:24,970 We looked at it when discussing method binding. 30 00:02:25,170 --> 00:02:31,290 It can be used to disassemble one or more class once and it can also take multiple command line options 31 00:02:31,290 --> 00:02:32,350 . 32 00:02:32,400 --> 00:02:37,890 For example if you include the option hyphen C it would print the byte code for each of them methods 33 00:02:37,940 --> 00:02:38,950 in the class. 34 00:02:39,600 --> 00:02:45,210 But if you can grab the option hyphen we are hyphen bobos then it would still print the byte code of 35 00:02:45,300 --> 00:02:46,490 each of the methods. 36 00:02:46,710 --> 00:02:53,370 But in addition it would also provide the dark constant boune on some additional information in case 37 00:02:53,370 --> 00:02:55,630 you want to view all the possible options. 38 00:02:55,830 --> 00:02:58,330 You can use hyphen help. 39 00:02:58,500 --> 00:03:05,860 So let's go ahead and use hyphen VI option on the Hello class as shown here under the bytecode. 40 00:03:07,000 --> 00:03:07,280 OK. 41 00:03:07,290 --> 00:03:08,940 Now here is the Hello class. 42 00:03:09,030 --> 00:03:10,710 So you're not going to look at that. 43 00:03:10,980 --> 00:03:14,590 So I'm just going to compile this first and then. 44 00:03:15,030 --> 00:03:18,810 So I'm just going to run the jalopy down with the hyphen B option. 45 00:03:18,870 --> 00:03:19,330 OK. 46 00:03:19,350 --> 00:03:22,540 And it's going to be hello dark class here. 47 00:03:22,680 --> 00:03:27,290 OK so we are running or we are running it on the classified. 48 00:03:27,810 --> 00:03:30,510 Now here is a byte code. 49 00:03:31,350 --> 00:03:36,080 And we can see that this is the method called by a byte code Fard the two string mentored. 50 00:03:36,320 --> 00:03:41,030 And similarly we have the bytecode for Main method here. 51 00:03:42,510 --> 00:03:48,750 And this is the bytecode for the for the constructor for which is which is inserted automatically the 52 00:03:48,750 --> 00:03:50,370 default constructor. 53 00:03:50,640 --> 00:03:55,180 And we're going to analyze each of them separately the constructor and the two methods. 54 00:03:55,440 --> 00:03:58,450 And then look at the byte code generated by them. 55 00:03:58,520 --> 00:04:00,290 And here is the constant pool. 56 00:04:00,450 --> 00:04:02,960 OK so this is the constant pool that's generated. 57 00:04:03,000 --> 00:04:06,220 It's this constant pool and all of this is there. 58 00:04:07,020 --> 00:04:10,490 And this was generated because of the hyphen v option. 59 00:04:10,700 --> 00:04:11,310 OK. 60 00:04:11,340 --> 00:04:14,910 And that is summertime information too and that's about it. 61 00:04:14,910 --> 00:04:19,870 So let's just go ahead and review each of the methods under constructor. 62 00:04:20,970 --> 00:04:26,430 Let's begin with looking at the bytecode generated for the constructor which was inserted automatically 63 00:04:26,430 --> 00:04:27,760 by the compiler. 64 00:04:28,200 --> 00:04:33,960 Now if you recall compiler when it auto inserts of default constructor it also ought to insert a statement 65 00:04:34,080 --> 00:04:38,330 for invoking the superclasses default constructor. 66 00:04:38,340 --> 00:04:41,820 And we know it's due to the constructor chaining process. 67 00:04:41,820 --> 00:04:45,560 The first two byte code instructions that you see here correspond to that statement. 68 00:04:45,780 --> 00:04:53,050 And let's see what they signify the first instruction here is in order underscores zero earlier in the 69 00:04:53,040 --> 00:04:59,160 bit I load instruction the prefix in this gives indicates an object reference. 70 00:04:59,550 --> 00:05:05,020 Well in the case of I load the prefix i indicates an in data type. 71 00:05:05,280 --> 00:05:09,000 Other than the small difference the operation performed is identical. 72 00:05:09,000 --> 00:05:14,580 That is it tigs the element at the specified index in the local variables array and pushes it onto the 73 00:05:14,580 --> 00:05:18,890 top of the upper and stock index because the index is zero. 74 00:05:20,070 --> 00:05:27,380 Not index zero we have this reference and that's because Kunstler is like an instance method unreleased 75 00:05:27,440 --> 00:05:32,740 because that for instance Medders the zeroth element in the local variables will be a reference to the 76 00:05:32,740 --> 00:05:36,760 current object that is this reference. 77 00:05:36,750 --> 00:05:43,030 The reason why we have this reference is because it allows the code in the method or the constructor 78 00:05:43,120 --> 00:05:45,790 to access any instance variables. 79 00:05:45,790 --> 00:05:52,270 Similarly if the current method or constructor is invoking another method or constructor in the same 80 00:05:52,260 --> 00:05:58,830 class or in a superclass then the this reference will be set in the local variables array corresponding 81 00:05:58,840 --> 00:06:07,210 to that method or constructor next instruction is an expression and it has one operand which is hash 82 00:06:07,210 --> 00:06:14,110 one getting a constructor in the superclass which is the object us apart from constructor. 83 00:06:14,230 --> 00:06:21,010 Invoke special instruction will also be generated when we are in working either a private method or 84 00:06:21,000 --> 00:06:25,650 when we are in Woking as superclass method we are the supercooled. 85 00:06:25,690 --> 00:06:32,170 So you can try that out just are a private matter and then and look at the byte code and you should 86 00:06:32,170 --> 00:06:35,640 be able to see and invoke special instruction. 87 00:06:35,670 --> 00:06:41,920 Now here we are referring the constructor as instance initialization method and that's because of JVM 88 00:06:41,920 --> 00:06:43,580 to do it. 89 00:06:43,770 --> 00:06:49,980 In fact what we are seeing here in this particular snapshot is an instance initialization method and 90 00:06:49,990 --> 00:06:53,710 such a method is inserted for each constructor in that class. 91 00:06:53,880 --> 00:07:00,190 But in addition to constructor called this instance initialization method will also initialize all the 92 00:07:00,190 --> 00:07:06,550 instance variables which in the source code might have been initialized outside of the constructor. 93 00:07:06,550 --> 00:07:09,320 Let me actually show that to you in the code. 94 00:07:10,440 --> 00:07:15,060 So let me go ahead and I had an instance variable here. 95 00:07:15,100 --> 00:07:19,780 Should I use it to 3 so it's initialized outside the constructor. 96 00:07:19,770 --> 00:07:21,350 In fact there is no constructor here. 97 00:07:21,510 --> 00:07:28,950 So let's just go ahead and compile this and then do a java command. 98 00:07:29,380 --> 00:07:32,980 Now let's look at the construct of code. 99 00:07:35,050 --> 00:07:37,530 As you can see here in the constructor called. 100 00:07:37,750 --> 00:07:42,820 So we have these three bytecode instructions in order underscore zero icons to underscore three and 101 00:07:42,880 --> 00:07:43,970 put free. 102 00:07:44,350 --> 00:07:49,370 So this is the code that is initializing the instance variable. 103 00:07:49,940 --> 00:07:50,940 So that's the thing. 104 00:07:50,950 --> 00:07:56,280 So in addition to the constructor code itself which is in this case in working the superclass constructor 105 00:07:56,950 --> 00:08:00,250 it also initializes the instance variables. 106 00:08:00,500 --> 00:08:08,290 How in this instance initialization method is also referenced using a special name called in it which 107 00:08:08,290 --> 00:08:12,400 you can see here in the common the name here in the signature. 108 00:08:12,400 --> 00:08:17,520 It still has the class name but in the constant bool it really did it for instance in it and we will 109 00:08:17,520 --> 00:08:19,020 see that in a moment. 110 00:08:19,680 --> 00:08:25,870 No come into the operation hash run it is simply an offset into the runtime constant pull of the current 111 00:08:25,870 --> 00:08:27,160 class. 112 00:08:27,190 --> 00:08:33,870 So let's look at the relevant section of the constant pull period is the constant Paulas sequential 113 00:08:33,900 --> 00:08:36,790 number but to highlight only the relevant part. 114 00:08:37,090 --> 00:08:43,360 I extracted only the relevant lines from the constant pull index one is basically a symbolic reference 115 00:08:43,360 --> 00:08:44,300 to him either. 116 00:08:44,500 --> 00:08:51,010 And in this case it's a constructor index one is referencing two other indices mind and pointy and nicks 117 00:08:51,000 --> 00:08:57,330 nine is referencing in index 28 which is a symbolic reference to the object us. 118 00:08:57,340 --> 00:09:00,020 Similarly in next one p is intended for rinsing. 119 00:09:00,050 --> 00:09:07,520 And this has been an 11 word 10 gives the method name which as mentioned earlier is especially in that 120 00:09:08,740 --> 00:09:14,180 index Levan holds information about what is called does matter descriptor which includes the matter 121 00:09:14,230 --> 00:09:16,910 parameters under it and type. 122 00:09:17,620 --> 00:09:19,370 In this case there are no parameters. 123 00:09:19,380 --> 00:09:25,650 We only have an empty promises when the return type is void which is indicated by the letter B. 124 00:09:26,640 --> 00:09:27,700 Know at runtime. 125 00:09:27,750 --> 00:09:33,600 When GBM and contest this in Rook's special instruction it performs a few actions. 126 00:09:33,700 --> 00:09:39,120 First it performs method resolution which in this case is a resolution of the instance initialization 127 00:09:39,120 --> 00:09:40,560 method. 128 00:09:40,680 --> 00:09:46,580 We know that this would entail replacing symbolic reference with direct reference. 129 00:09:47,190 --> 00:09:53,430 Next JVM creates a new stack frame for the instance initialization method and pushes it onto the stack 130 00:09:53,550 --> 00:09:55,530 . 131 00:09:56,060 --> 00:10:02,920 Nexted pops this reference from the operand stack and started index 0 of the local variables array of 132 00:10:02,910 --> 00:10:04,610 the newly created stack. 133 00:10:05,110 --> 00:10:10,270 So the reason for having Elorde underscores zero was for storing this reference variable on the stack 134 00:10:10,720 --> 00:10:12,830 so that Iraq's Special can use it next. 135 00:10:13,950 --> 00:10:17,340 Know the instance initialization method is ready to be in work. 136 00:10:18,060 --> 00:10:23,470 And so the program counter will be set with the first instruction in the instance initialization method 137 00:10:23,560 --> 00:10:25,100 of the superclass. 138 00:10:25,420 --> 00:10:30,370 So the execution continues from the first instruction. 139 00:10:30,370 --> 00:10:31,940 So that was a constructor court. 140 00:10:32,010 --> 00:10:35,380 I need an artist to review the Invoke special instruction. 141 00:10:35,380 --> 00:10:38,360 Now let's look at the main method byte order. 142 00:10:38,500 --> 00:10:42,530 The first statement in the main method creates a new instance of Hello. 143 00:10:43,000 --> 00:10:47,610 Now this statement generates the first three bytecode instructions that you'll see here. 144 00:10:47,620 --> 00:10:49,930 So let's start to make sense of this mapping. 145 00:10:50,230 --> 00:10:56,290 The first bytecode instruction which includes a opcode new creates an instance of hello on pushes the 146 00:10:56,380 --> 00:10:59,100 object reference onto the operand stack. 147 00:10:59,770 --> 00:11:06,450 So at this point only the object is created but for it to be fully formed we need to also in the constructor 148 00:11:06,450 --> 00:11:06,930 code. 149 00:11:07,760 --> 00:11:13,580 For that we know that we need an in special instruction but that's the third instruction. 150 00:11:13,870 --> 00:11:16,120 And there is a second instruction. 151 00:11:16,120 --> 00:11:22,030 The second instruction is doop and it basically makes a copy of the item which is on the top of the 152 00:11:22,030 --> 00:11:26,420 operand stack and also stores it on the top of the opening stack. 153 00:11:26,420 --> 00:11:32,680 Now we need to make this copy as one of the things in walks Bishan does is to pop the top element from 154 00:11:32,680 --> 00:11:39,010 the operand stack and store it as the zero element and the local variables array off the new frame that 155 00:11:39,010 --> 00:11:40,380 it is going to create. 156 00:11:41,020 --> 00:11:46,660 But if we don't have the loop instruction then invoke special would Bob the only object reference that 157 00:11:46,660 --> 00:11:53,050 we have and with that we will not have a way to end with the string method which is the second statement 158 00:11:53,050 --> 00:11:54,380 of the main method. 159 00:11:54,790 --> 00:11:57,970 So we do need the instruction. 160 00:11:57,970 --> 00:12:03,960 Next the following three bytecode instructions correspond to the second statement the instruction A-story 161 00:12:03,980 --> 00:12:08,700 must go to one Bob's the hallowed reference from the operand stack unstarted at. 162 00:12:08,730 --> 00:12:11,320 And next one in the local variables at it. 163 00:12:11,680 --> 00:12:18,310 But if you are storing it index one at index 0 we have the only method parameter off the main method 164 00:12:18,850 --> 00:12:20,810 which is a reference to the string theory. 165 00:12:21,380 --> 00:12:24,010 Next a lot underscore one does the exact opposite. 166 00:12:24,010 --> 00:12:30,160 That is it moves the element back to the stack from the local variables adding finally in-work which 167 00:12:30,160 --> 00:12:32,860 will BOP's the hallowed reference from the stack. 168 00:12:33,070 --> 00:12:36,190 And it also in wig's that boosting method. 169 00:12:36,460 --> 00:12:42,160 Know if you recall we discussed in work which will indeed did in the method by adding lesson which was 170 00:12:42,160 --> 00:12:45,380 covered in the inheritance and polymorphism chapter. 171 00:12:45,720 --> 00:12:51,580 That lesson we said that for instance methods met that signature binding happens at compile time itself 172 00:12:52,540 --> 00:12:56,890 but the method implementation binding happens only at runtime. 173 00:12:56,950 --> 00:13:02,390 That's because with instance methods we do not know which method to work until runtime. 174 00:13:02,410 --> 00:13:05,660 That is it is dependent on the object in context. 175 00:13:05,680 --> 00:13:11,830 For example here you can see that at compile time method signature is bonded to the string method in 176 00:13:11,830 --> 00:13:13,130 that object class. 177 00:13:13,750 --> 00:13:20,470 But at runtime the string method in the Hello class will be executed just like in the case of in Woakes 178 00:13:20,470 --> 00:13:21,060 mission. 179 00:13:21,160 --> 00:13:27,670 Let's also review the steps performed when in which what is encountered also autoloader the pop instruction 180 00:13:27,760 --> 00:13:32,310 which is the next one simply pops the top element on the stack. 181 00:13:32,470 --> 00:13:35,250 It has a value returned by the spring mother. 182 00:13:35,530 --> 00:13:41,260 Finally the return instruction at the end simply implies right as written. 183 00:13:41,260 --> 00:13:48,190 When JVM encounters in-work what will it Fussball force method resolution which in our case is the resolution 184 00:13:48,190 --> 00:13:51,180 of the string method and the object us. 185 00:13:51,460 --> 00:13:58,430 This could simply mean pointing to an index in the metric table of the object class and the element 186 00:13:58,420 --> 00:14:02,300 there would point to the actual string method code. 187 00:14:02,970 --> 00:14:08,130 But the method to be executed is dependent on the actual object for that runtime. 188 00:14:08,260 --> 00:14:13,250 We know that the operand stack has a reference to this actual object. 189 00:14:13,690 --> 00:14:16,180 So JVM performs a lookup operation here. 190 00:14:16,630 --> 00:14:22,210 It starts with the reference on the operand stack and follows it to the object on heap and then into 191 00:14:22,210 --> 00:14:23,110 its method table. 192 00:14:23,140 --> 00:14:30,730 In the metro area the actual implementation will be in the metal table at the next phone doing the matter 193 00:14:30,730 --> 00:14:32,510 resolution step. 194 00:14:32,740 --> 00:14:38,710 So the index number found during method resolution step then applies here in them at the table corresponding 195 00:14:38,710 --> 00:14:40,570 to that Holocaust. 196 00:14:40,600 --> 00:14:45,940 We discussed this when discussing method tables and we said that we are able to use the same index member 197 00:14:46,270 --> 00:14:52,870 as a reference type of the variable is a class not an interface does not that using Mathare table in 198 00:14:52,870 --> 00:14:59,280 such a way for a method implementation binding is entirely up to JVM implementation. 199 00:14:59,320 --> 00:15:02,280 Some implementations might do it in a different way too. 200 00:15:02,890 --> 00:15:05,870 At this point we know the method to be executor. 201 00:15:06,940 --> 00:15:13,450 So the next three steps are identical to what we saw in works Bishan that is create a new stack frame 202 00:15:13,450 --> 00:15:16,750 for boosting that and push it onto the stack. 203 00:15:16,820 --> 00:15:23,110 But this reference from operand stack unstarted that index zero of the local variables array of the 204 00:15:23,170 --> 00:15:30,490 new stack frame find a decent program counter to the first instruction of the new method. 205 00:15:30,490 --> 00:15:35,470 Now here is a last snapshot which corresponds to a boosting method heading through quota system not 206 00:15:35,470 --> 00:15:38,620 all that Prindle and segment which prints a string hello. 207 00:15:38,620 --> 00:15:44,440 It also returns a string literal Hello no in the expression system that called We know that system is 208 00:15:44,440 --> 00:15:48,660 a class an out is a static variable defined in that class. 209 00:15:48,940 --> 00:15:55,510 The type of code really obvious but in stream from the Java dot or package sprint and method is invoked 210 00:15:55,510 --> 00:15:57,510 on the reference variable. 211 00:15:58,030 --> 00:16:05,170 So we need to forestall the old reference variable in the opening stack and this is done by get static 212 00:16:05,170 --> 00:16:06,980 instruction. 213 00:16:07,210 --> 00:16:15,400 So it basically pushes the value of all credible onto the open and stack next instruction LTC gets the 214 00:16:15,400 --> 00:16:21,140 reference to the string literal Hello from the runtime constant pull on pushes it onto the open and 215 00:16:21,140 --> 00:16:22,880 stack. 216 00:16:23,590 --> 00:16:24,970 Finally invoke what you want. 217 00:16:24,980 --> 00:16:28,290 BOP's both the string literal hello and the reference. 218 00:16:28,440 --> 00:16:30,500 And in walks the print method. 219 00:16:30,710 --> 00:16:34,050 Then the string literal hello as the argument. 220 00:16:34,830 --> 00:16:42,160 No the last two instructions correspond to retaining the string Hello Azmat that's output under instruction 221 00:16:42,170 --> 00:16:42,220 . 222 00:16:42,280 --> 00:16:47,080 Arrogance simply implies that we are returning an object reference. 223 00:16:47,110 --> 00:16:53,980 There are other variations we're primitive types like I've written for returning an end date type. 224 00:16:54,100 --> 00:16:55,430 So that's about it. 225 00:16:55,430 --> 00:17:01,270 They look at both in word spatial and in word what you're willing to put in was started in detail and 226 00:17:01,270 --> 00:17:02,520 method by adding lesson. 227 00:17:02,740 --> 00:17:05,520 And it is related to in working static methods. 228 00:17:05,710 --> 00:17:12,370 You can review that read you again if needed another instruction you know as in rock interface in our 229 00:17:12,370 --> 00:17:18,630 program the reference type is a class object and the actual type of the object was hello. 230 00:17:19,030 --> 00:17:24,610 But if the reference type was an interface then it would have generated and invoke interface instruction 231 00:17:24,620 --> 00:17:25,050 . 232 00:17:25,630 --> 00:17:32,950 So you can try it out by making the Hello class implement an interface Resorcin section includes a pointer 233 00:17:32,950 --> 00:17:40,390 to the JVM specification page that includes detail information about all the bytecode instructions. 234 00:17:40,390 --> 00:17:45,730 So if you want to get a more in-depth understanding of any of the instructions that recolored or if 235 00:17:45,730 --> 00:17:50,850 you want to learn about any new instruction then you can refer to this page. 236 00:17:51,010 --> 00:17:52,450 So that's about it. 237 00:17:52,450 --> 00:17:56,620 Thanks and I hope you'll enjoy learning about the bytecode instructions.