JVM bytecode - The secret language behind Java and Scala
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The JVM bytecode is a set of low-level instructions that are executed by the JVM. Bytecode has no variables, binary logical operators, or loop constructs like other languages. Instead, it employs registers and stacks to hold values and uses jump instructions to implement logical operators and loops. Bytecode is similar to assembly code and allows Java and Scala code to run on the JVM.
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JVM bytecode - The secret language behind Java and Scala
- 1. JVM bytecode The secret language behind Java and Scala
- 2. About Me Writing code for the past ~15 years. 1. Dev at IDF intl 2. Team lead, architect at IAI Space Industries 3. CEO at VisualTao 4. Director - AutoCAD Web & Mobile, GM Autodesk IL 5. CEO at Takipi
- 3. Overview 1. What is bytecode 2. The 3 biggest differences between source code and bytecode 3. 5 things you should know about bytecode 4. Practical uses
- 4. What is bytecode? A set of low-level instructions to be executed by the JVM. ~200 instruction types, each ~1-2 bytes in size. Some instructions are very similar to Java, some completely different.
- 5. Bytecode is very similar to Assembly (That’s why we avoid it…) assembly - > exec file -> OS bytecode -> .class file -> JVM .cpp -> g++ -> exec file -> OS .java -> JavaC -> .class file -> JVM .scala-> ScalaC -> .class file -> JVM
- 6. The 3 biggest differences between source and byte code
- 7. 1. No variables Bytecode employs an Assembly-like register stack known as the locals stack to hold variables. Values of fields, functions and of binary operations (+, -, * ..) are held in a stack known as the operand stack.
- 8. public getField() : double L0 public class LocalVars ALOAD 0: this { GETFIELD NoLocalVars.state : boolean private int intField; IFEQ L1 private double doubleField; L2 ALOAD 0: this private boolean state; GETFIELD NoLocalVars.intField : int ICONST_1 public double getField() IADD { ISTORE 1 if (state) L3 { GETSTATIC System.out : PrintStream ILOAD 1: a int a = intField + 1; INVOKEVIRTUAL PrintStream.println(int) : void return a; L4 } ILOAD 1: a else I2D { DRETURN double b = doubleField + 1; L1 return b; ALOAD 0: this } GETFIELD NoLocalVars.doubleField : double } DCONST_1 } DADD DSTORE 1 L5 GETSTATIC System.out : PrintStream DLOAD 1: b Notes INVOKEVIRTUAL PrintStream.println(double) : void L6 Notice how the same register slot (1) is DLOAD 1: b re-used between blocks for different variables. DRETURN L7 The variable meta-data table describes the LOCALVARIABLE this NoLocalVars L0 L7 0 mappings between registers and source code LOCALVARIABLE a int L3 L1 1 variables LOCALVARIABLE b double L5 L7 1 MAXSTACK = 4 MAXLOCALS = 3
- 9. 2. No binary logical operators No built-in support for &&, ||, ^ Compilers implement these using jump instructions.
- 10. public and(boolean, boolean) : void public class NoLogicalOperators L0 { ILOAD 1: a public void and(boolean a, boolean b) { IFEQ L1 if (a && b) { ILOAD 2: b System.out.println("its true"); IFEQ L1 } L2 } GETSTATIC System.out : PrintStream LDC "its true" public void or(boolean a, boolean b) { INVOKEVIRTUAL PrintStream.println(String) : void if (a || b) { L1 System.out.println("its true"); RETURN L3 } } } public or(boolean, boolean) : void L0 ILOAD 1: a IFNE L1 ILOAD 2: b IFEQ L2 L1 GETSTATIC System.out : PrintStream Notes LDC "its true" INVOKEVIRTUAL PrintStream.println(String) : void Notice how both && and || operators are L2 implemented using jump instructions who evaluate RETURN the last value if the operand stack L3
- 11. public class NoLogicalOperators public orX2(boolean, boolean, boolean, boolean) : void { L0 public void orX2(boolean a, boolean b, ILOAD 1: a boolean c, boolean d) { IFNE L1 if ((a || b) && (c || d)) ILOAD 2: b { IFEQ L2 System.out.println("its true"); L1 } ILOAD 3: c } IFNE L3 } ILOAD 4: d IFEQ L2 L3 GETSTATIC System.out : PrintStream LDC "its true" INVOKEVIRTUAL PrintStream.println(String) : void L2 RETURN L4 Notes For composite ||, && conditions compilers will generate multiple jump combinations
- 12. 3. No loop constructs There’s no built-in support for while, for, for-each loops. Compilers implement these using jump instructions.
- 13. public class Loops public forLoop(int) : void { L0 public void forLoop(int n) { ICONST_0 for (int i = 0; i < n; i++) { ISTORE 2 System.out.println(i); L1 } GOTO L2 } L3 } GETSTATIC System.out : PrintStream ILOAD 2: i INVOKEVIRTUAL PrintStream.println(int) : void L4 IINC 2: i 1 L2 ILOAD 2: i ILOAD 1: n IF_ICMPLT L3 L5 RETURN L6 LOCALVARIABLE this Loops L0 L6 0 LOCALVARIABLE n int L0 L6 1 LOCALVARIABLE i int L1 L5 2 Notes A for loop is implemented using a conditional jump instruction comparing i and n
- 14. public class Loops public whileLoop(int) : void { L0 public void whileLoop(int n) ICONST_0 { ISTORE 2 int i = 0; L1 GOTO L2 while (i < n) L3 { GETSTATIC System.out : PrintStream System.out.println(i); ILOAD 2: i i++; INVOKEVIRTUAL PrintStream.println(int) : void } L4 } IINC 2: i 1 } L2 ILOAD 2: i ILOAD 1: n IF_ICMPLT L3 L5 RETURN L6 LOCALVARIABLE this Loops L0 L6 0 LOCALVARIABLE n int L0 L6 1 LOCALVARIABLE i int L1 L6 2 Notes This while loop is also implemented using a conditional jump instruction comparing i and n. The bytecode is nearly identical to the previous loop.
- 15. public class Loops { public forEachLoop(List) : void public void forEachLoop(List<String> strings) { L0 for (String s : strings) { ALOAD 1: strings System.out.println(s); INVOKEINTERFACE List.iterator() : Iterator } ASTORE 3 } GOTO L1 } L2 ALOAD 3 INVOKEINTERFACE Iterator.next() : Object CHECKCAST String ASTORE 2 L3 GETSTATIC System.out : PrintStream ALOAD 2: s INVOKEVIRTUAL PrintStream.println(String) : void L1 ALOAD 3 INVOKEINTERFACE Iterator.hasNext() : boolean IFNE L2 L4 RETURN L5 LOCALVARIABLE this Loops L0 L5 0 LOCALVARIABLE strings List L0 L5 1 Notes // declaration: java.util.List<java.lang.String> LOCALVARIABLE s String L3 L1 2 A for-each loop is generated by the javaC compiler } by jumping against the hasNext() method. The result bytecode is unaware of the for-each construct. Also notice how register 3 is used to hold the iterator
- 16. 5 Things you should know about bytecode that affect everyday programming
- 17. 1. No String support Like in C, there’s no built-in support for strings, only char arrays. Compilers usually use StringBuilder to compensate. No penalty for concatenating different data types
- 18. public class ImplicitStrings // access flags 0x1 { public toString(int, int) : String public String toString(int a, int b) L0 { NEW StringBuilder DUP String c = "Hello " + a + "World" + b; LDC "Hello " return c; INVOKESPECIAL StringBuilder.<init>(String) : void } ILOAD 1: a } INVOKEVIRTUAL StringBuilder.append(int) : StringBuilder LDC "World" INVOKEVIRTUAL StringBuilder.append(String) : StringBuilder ILOAD 2: b INVOKEVIRTUAL StringBuilder.append(int) : StringBuilder INVOKEVIRTUAL StringBuilder.toString() : String ASTORE 3 L1 ALOAD 3: c ARETURN L2 LOCALVARIABLE this ImplicitStrings L0 L2 0 LOCALVARIABLE a int L0 L2 1 LOCALVARIABLE b int L0 L2 2 LOCALVARIABLE c String L1 L2 3 Notes JavaC uses java.lang.StringBuilder to combine (+)strings. Different overloads of the .append() method are used to concat different data types.
- 19. public toString1(int, int) : String public class ImplicitStrings L0 { NEW StringBuilder public String toString1(int a, int b) DUP { LDC "Hello" String c; INVOKESPECIAL StringBuilder.<init>(String) : void ILOAD 1: a INVOKEVIRTUAL StringBuilder.append(int) : StringBuilder c = "Hello" + a; INVOKEVIRTUAL StringBuilder.toString() : String c += "World" + b; ASTORE 3 L1 return c; NEW StringBuilder } DUP } ALOAD 3: c INVOKESTATIC String.valueOf(Object) : String INVOKESPECIAL StringBuilder.<init>(String) : void LDC "World" INVOKEVIRTUAL StringBuilder.append(String) : StringBuilder ILOAD 2: b INVOKEVIRTUAL StringBuilder.append(int) : StringBuilder INVOKEVIRTUAL StringBuilder.toString() : String ASTORE 3: c L2 ALOAD 3: c ARETURN L3 Notes While this code is identical to the previous example in terms of functionality, there’s a performance penalty to note as 2 StringBuilders are constructed
- 20. 2. Only 4 primitive types Bytecode only operates on 4 primitives types ( int, float, double, long) vs. the 8 Java primitives. Doesn’t operate on char, bool, byte, short (treated as ints)
- 21. public mulByeShort(byte, short) : void public class BytecodePrimitives L0 { GETSTATIC System.out : PrintStream public void mulByeShort(byte b, short c) ILOAD 1: b { ILOAD 2: c System.out.println(b * c); IMUL } INVOKEVIRTUAL PrintStream.println(int) : void L1 public void mulInts(int b, int c) RETURN { L2 LOCALVARIABLE this B_BytecodePrimitives L0 L2 0 System.out.println(b * c); LOCALVARIABLE b byte L0 L2 1 } LOCALVARIABLE c short L0 L2 2 } public mulInts(int, int) : void L0 GETSTATIC System.out : PrintStream ILOAD 1: b ILOAD 2: c IMUL INVOKEVIRTUAL PrintStream.println(int) : void L1 RETURN L2 LOCALVARIABLE this B_BytecodePrimitives L0 L2 0 LOCALVARIABLE b int L0 L2 1 LOCALVARIABLE c int L0 L2 2 Notes Notice how the bytecode for these 2 methods is identical, regardless of the difference in var types
- 22. public class BytecodePrimitives public printIfTrue(boolean) : void { L0 public void printIfTrue(boolean b) ILOAD 1: b { IFEQ L1 L2 if (b) GETSTATIC System.out : PrintStream { LDC "Hi" System.out.println("Hi"); INVOKEVIRTUAL PrintStream.println(String) : void } L1 } RETURN L3 public void printIfN0(int i) LOCALVARIABLE this B_BytecodePrimitives L0 L3 0 { LOCALVARIABLE b boolean L0 L3 1 if (i != 0) { public printIfN0(int) : void System.out.println("Hi"); L0 ILOAD 1: i } IFEQ L1 } L2 } GETSTATIC System.out : PrintStream LDC "Hi" INVOKEVIRTUAL PrintStream.println(String) : void L1 RETURN L3 LOCALVARIABLE this B_BytecodePrimitives L0 L3 0 Notes LOCALVARIABLE i int L0 L3 1 The same observation is also true when evaluating conditions. See how both boolean and int operations are treated the same.
- 23. 3. Using nested classes? Compilers will add synthetic $this fields. If you’re not making calls to your outer-class - don’t forget to add a static modifier.
- 24. public class C_NestedClasses public class NestedClasses { { public class C_NestedClasses$NestedClass public class NestedClass { { final C_NestedClasses this$0 } public <init>(C_NestedClasses) : void public static class StaticNestedClass L0 { ALOAD 0: this ALOAD 1 } PUTFIELD C_NestedClasses$NestedClass.this$0 : C_NestedClasses ALOAD 0: this } INVOKESPECIAL Object.<init>() : void RETURN L1 LOCALVARIABLE this C_NestedClasses$NestedClass L0 L1 0 } public class C_NestedClasses$StaticNestedClass { public <init>() : void L0 ALOAD 0: this INVOKESPECIAL Object.<init>() : void Notes RETURN L1 The NestedClass inner-class has an implicit $this0 created for him, and assigned in the constructor. }
- 25. Using nested classes (2)? Try and avoid implicitly creating bridge methods by invoking private members (use protected)
- 26. static access$0(D_BridgeMethods) : int public class BridgeMethods L0 { ALOAD 0 private int member; GETFIELD D_BridgeMethods.member : int IRETURN public class BridgeMethodClass MAXSTACK = 1 { MAXLOCALS = 1 public void printBridge() { public printBridge() : void System.out.println(member); L0 } GETSTATIC System.out : PrintStream } ALOAD 0: this GETFIELD D_BridgeMethods$BridgeMethodClass.this$0 : D_BridgeMethods } INVOKESTATIC D_BridgeMethods.access$0(D_BridgeMethods) : int INVOKEVIRTUAL PrintStream.println(int) : void L1 RETURN L2 LOCALVARIABLE this D_BridgeMethods$BridgeMethodClass L0 L2 0 Notes When a private field or method is invoked javaC will add synthetic bridge methods to allow the internal class to access private members of its outer-class
- 27. 4. Boxing and unboxing Boxing is added by the Java/Scala compiler. There’s no such concept in bytecode or in the JVM. Watch out for NullPointerExceptions
- 28. public printSqr(int) : void L0 ILOAD 1: a ISTORE 2 public class Boxing L1 { ILOAD 1: a public void printSqr(int a) INVOKESTATIC Integer.valueOf(int) : Integer { ASTORE 3 int a1 = a; L2 GETSTATIC System.out : PrintStream Integer a2 = a; ILOAD 2: a1 ALOAD 3: a2 System.out.println(a1 * a2); INVOKEVIRTUAL Integer.intValue() : int } IMUL INVOKEVIRTUAL PrintStream.println(int) : void L3 public void check(Integer i) RETURN { L4 if (i == 0) LOCALVARIABLE this E_Boxing L0 L4 0 { LOCALVARIABLE a int L0 L4 1 System.out.println("zero"); LOCALVARIABLE a1 int L1 L4 2 } LOCALVARIABLE a2 Integer L2 L4 3 } } public check(Integer) : void L0 ALOAD 1: i INVOKEVIRTUAL Integer.intValue() : int IFNE L1 L2 Notes GETSTATIC System.out : PrintStream LDC "zero" Notice how javaC implicitly invokes the various INVOKEVIRTUAL PrintStream.println(String) : void java.lang.Integer methods. L1 RETURN Other compilers, such as scalaC, use their own L3 LOCALVARIABLE this E_Boxing L0 L3 0 boxed types LOCALVARIABLE i Integer L0 L3 1 }
- 29. 5. 3 bytecode myths 1. Bytecode supports multiple inheritance 2. Illegal bytecode can crash the JVM (it’s blocked by the JVM verifier) 3. Low-level bytecode can operate outside the JVM sandbox
- 30. 3 Main bytecode uses 1. Building a compiler 2. Static analysis 3. JVM bytecode instrumentation
- 31. Building a “better Java” through Scala and ScalaC. A new OO/functional language which compiles into standard JVM bytecode. Transparent from the JVM’s perspective.
- 32. Takipi - Overview Explain the cause of server exceptions, latency and unexpected code behavior at scale. Help R&D teams solve errors and downtime in production systems, without having to re-deploy code or sift through log files.
- 33. Takipi & bytecode 1. Index bytecode in the cloud. 2. When an exception occurs, query the DB to understand which variables, fields and conditions are causing it. 3. Instrument new bytecode to log the values causing the exception. 4. Present a “story” of the exception to the developer.
- 34. Thanks! Join our private beta - takipi.com/signup [email protected] @takipid (tweeting about Java, Scala, DevOps and Cloud)